At least 40 people have been killed as fighting on the ground between regime forces and rebels resumed in the northwestern Syrian province of Idlib on Thursday following a week of air strikes.
The violence further buried a ceasefire announced by Russia on Sunday in agreement with Turkey. Both nations support different sides in the conflict.
The Syrian government launched a ground offensive that killed 39 people on Thursday and defying a truce deal brokered by Turkey and Russia.
The Syrian Observatory for Human Rights said the intense fighting between pro-government forces and extremists started late Wednesday in the key town of Maaret Al Numan.
The area is a key target of the Syrian government's latest military offensive to re-take the province.
Government and allied forces took two villages in their advance, the Britain-based war monitor said.
At least 22 anti-government fighters were killed, most of them members of Hayat Tahrir Al Sham, a group that includes fighters from the former Al Qaeda affiliate in Syria.
Seventeen government troops and allied militia were also killed in the fighting, said Rami Abdel Rahman, the head of the Observatory.
Nearly nine years into the conflict, protests against the government are still held in some of the province's towns.
In the city of Idlib itself, 19 civilians were killed in air strikes on Wednesday, shattering the truce brokered by Moscow and rebel backer Ankara.
The raids hit a vegetable market and repair shops in Idlib, capital of the rebel-held province of the same name, said the Syrian Observatory for Human Rights.
The fighting has prompted around 350,000 Syrians, mostly women and children, to flee thier homes since early December, the UN announced on Thursday.
"This latest wave of displacement compounds an already dire humanitarian situation on the ground in Idlib," David Swanson, Amman-based UN regional spokesman for Syria, told Reuters.
Russian and Syrian jets resumed bombing of civilian areas in the opposition enclave two days after a ceasefire agreed between Turkey and Russia formally took effect on Sunday.
UN officials said earlier this month the humanitarian crisis had worsened, with thousands of civilians on the run in Idlib province on top of close to 400,000 people who fled earlier bouts of fighting to the safety of camps near the Turkish border.
The latest offensive has brought the Russian-steered military campaign closer to heavily populated parts of Idlib province, where nearly 3 million people are trapped, according to the United Nations.
