The Syrian army has sent troops to "confront the Turkish aggression" in the north of the country where Ankara is battling Kurdish-led forces, the state news agency SANA said on Sunday.
A Syrian Kurdish official and a war monitor also said that Syrian government forces were poised to enter Kurdish-controlled towns, from which US troops are withdrawing amid the Turkish offensive.
The Kurdish official would not disclose details but said a deal with Kurdish forces through Russia has been reached to deploy government forces in certain border towns. The official spoke on condition of anonymity because an official announcement has not yet been made.
The Syrian Observatory for Human Rights said the deal covered the towns of Kobane and Manbij. US troops had been deployed in the towns after they were cleared of Islamic State militants in 2015.
"Syrian... army units move north to confront Turkish aggression on Syrian territory," SANA said without giving further details.
The Syrian government move came as the US announced it was considering imposing "powerful sanctions" on Turkey, days after Donald Trump's decision to pull back some US forces in northern Syria.
Donald Trump's decision to pull out of the area cleared the way for Turkey to send its forces into the country and attack American-allied Kurdish militias. Human rights groups have appealed for the international community to take action as they accused Turkish forces and their proxies of killing up to 60 civilians since the cross-border assault began last Wednesday.
In series of tweets, Mr Trump said he was working with Congress on imposing "powerful sanctions" on Turkey, adding there is "great consensus" for the action. Minutes later, Senator Lindsey Graham also tweeted, confirming strong bipartisan support for the plan.
Treasury Secretary Steven Mnuchin said the US has full authority to impose sanctions quickly on Turkey that could include shutting down all US dollar transactions with the country’s entire government, which is something the administration is considering.
"We are ready to go on a moment's notice to put on sanctions," Mr Mnuchin said on ABC's This Week on Sunday. "These sanctions could be starting small, they could be maximum pressure which would destroy the Turkish economy."
At least 26 civilians were killed on Sunday in northeastern Syria, the UK-based Syrian Observatory for Human Rights said.
Among the casualties were 10 people killed in a Turkish air strike that hit a convoy of vehicles carrying civilians and journalists.
France 2 television journalist Stephanie Perez wrote on Twitter that she was travelling in the convoy that was hit.
"Our team is safe but colleagues have died," Ms Perez wrote.
The UN says the violence has forced 130,000 people to flee their homes.
Also on Sunday, US Defence Secretary Mark Esper said his country was poised to evacuate all of its troops from northern Syria as the fighting intensfies.
"In the last 24 hours, we learned that [the Turks] likely intend to extend their attack further south than originally planned, and to the west," Mr Esper said in an interview with "Face the Nation" on CBS.
"We also have learned in the last 24 hours that the ... SDF are looking to cut a deal, if you will, with the Syrians and the Russians to counterattack against the Turks in the north."
Mr Esper called the situation "untenable" for US forces, saying that he spoke with Mr Trump last night, and that the president directed the US military to "begin a deliberate withdrawal of forces from northern Syria."
Several European countries have suspended arms sales to Turkey following the incursion, including Germany, France and Finland. Also on Sunday, German Chancellor Angela Merkel told Turkey's President Recep Tayyip Erdogan to bring to an immediate halt the military operation in northern Syria.
"The Chancellor advocated an immediate end to the military operation," in a phone call, a spokeswoman for Ms Merkel said in a statement. The call took place at Mr Erdogan's request, she added.
The news of a further US withdrawal cam as Kurdish authorities said 785 foreign women and children affiliated with ISIS have fled from a detention camp in northeastern Syria on Sunday after Turkish forces shelled the site in the five-day offensive.
The US-backed, Kurdish-led Syrian Democratic Forces (SDF) operate several detention camps and prisons holding tens of thousands of ISIS fighters and families after the campaign against the terror group.
In the days ahead of Turkey’s offensive last Wednesday, authorities warned that the chaos and fighting could allow detainees to break out of prisons and camps and for the group to stage a comeback.
In an apparent reference to Turkish-backed Syrian rebels, the Kurdish administration said in a statement that "mercenaries" had attacked the camp where "Daesh elements" – a reference to ISIS – in turn attacked camp guards and opened the gates.
The centre, near the town of Ain Issa close to the frontlines, holds families of ISIS fighters.
Many of the normally more than 700 guards who secure the facility that houses 12,0000 displaced people have been called away to fight on the frontlines against Turkey and its allies.
"The guarding is very weak now," SDF official Marvan Qamishlo said, adding that just 60 or 70 security personnel remain.
He said that, ideally, 1,500 guards would be needed to fully staff the site, but added, “We don't have this sufficient number."
Advances by Turkish forces and their Syrian rebel allies have compounded the concerns of security personnel in the camp, giving rise to fears the site could be encircled, Mr Qamishlo said.
"What is a security person going to do? They are not special forces or SDF."
Save the Children released a statement expressing deep concern about reports hundreds of women and children with links to ISIS had fled an annexe of Ain Issa camp after the shelling.
The annexe was home to 249 women and 700 children linked to ISIS, Save the Children said.
“The reports suggest it is now completely empty of foreign women, and foreign masked men on motorbikes are circling the camp,” the NGO reported, adding that there is a danger that children of foreign nationals could now be lost in the chaos.
“Once again, we urgently call on foreign governments to repatriate their nationals while they can. The opportunity is quickly slipping away,” Save the Children Syria Response Director Sonia Khush said.
“We heard reports that the authorities on the ground took some of the foreign women to another location, but many have fled and some are unaccounted for,” Ms Khush added.
Save the Children added that Ain Isssa, home to approximately 25,000 people, “is completely empty as residents flee to safer areas.”
The SDF accused Turkey-backed rebels of killing a Kurdish politician in a road ambush on Saturday. The rebel force denied it, saying it had not advanced that far.
The Syrian Observatory said Turkey-backed groups had killed nine civilians on the road, including Hervin Khalaf, co-chair of the secular Future Syria Party.
The Observatory said, "the nine civilians were executed at different moments south of the town of Tal Abyad."
Local Kurdish authorities named the Kurdish political leader as Hevrin Khalaf, and said her driver was also among those killed.
Ms Khalaf, 35, was "taken out of her car during a Turkish-backed attack and executed by Turkish-backed mercenary factions", the political arm of the Kurdish-led Syrian Democratic Forces said.
"This is clear evidence that the Turkish state is continuing its criminal policy towards unarmed civilians," it said.
Mutlu Civiroglu, a specialist in Kurdish politics, described her death as a "great loss".
"She had a talent for diplomacy, she used to always take part in meetings with the Americans, the French, the foreign delegations."
Ms Khalaf was the secretary-general of the Future Syria Party.
The killing also drew criticism from inside Turkey.
“The execution of a civilian lying on the ground is terrible. It is a brutality to present it as a successful operation. It is also a disgrace [to say] that it is [not related] to our army. The groups that perform this atrocity are barbarians who have nothing to do with mehmetçik [soldiers of the Turkish army]. I strongly condemn [it],” said Mustafa Yeneroglu, a politician in Mr Erdogan’s AK Party tweeted.
Along the front lines, Turkish forces and Syrian rebels entered Suluk, some 10 kilometres from Turkey's border, the Syrian Observatory for Human Rights, a UK based war monitor, said on Sunday.
Turkey's state-owned Anadolu news agency said its rebels seized complete control of Suluk. But the SDF's Mr Qamishlo said their forces had repelled the attack and were still in control.
Suluk is southeast of the Syrian border town of Tel Abyad, one of the two main targets in the incursion, which was bombarded by Turkish howitzers on Sunday afternoon, a witness in the neighbouring Turkish town of Akcakale said.
The SDF on Saturday urged the US-led coalition to close air space to Turkish jets, saying SDF fighters were "being martyred by Turkish warplanes in front of the eyes of the allies".