Syrian government troops have intensified shelling in the southern province of Quneitra, which borders the Israeli-annexed Golan Heights, on Sunday and seized part of the area, a war monitor said.
"The bombardment started from 3am [local time] and continued for the last six hours," said Rami Abdel Rahman, head of the UK-based Syrian Observatory for Human Rights.
"Around 800 missiles and shells have been fired against several areas in the province and fierce fighting continues on the ground between regime forces and rebels.
“Bombardments have also hit areas in [neighbouring] Deraa province.”
The war monitor and rebels said on Sunday that they believe the jets that bombed Quneitra were Russian, adding it was the first such aerial strike in a year.
Mr Abdel Rahman said that forces loyal to Syrian President Bashar Al Assad have retaken the area of Mashara.
He also said that Syrian rebels and their relatives began evacuating Deraa on Sunday. AFP reported that hundreds of fighters and a few of their family members boarded around 15 buses which had then set off.
"Buses began moving from the gathering point towards the edge of the city to be searched," said Mr Abdel Rahman.
The transfers were part of a ceasefire deal brokered by government ally Russia, and came after more than two weeks of ferocious bombardment of the city.
The monitor said an estimated 1,400 people were expected to be evacuated on Sunday. That would pave the way for the entire city to come under government control in accordance with the handover deal.
Syrian state television confirmed the transfers had begun, broadcasting images of white buses they said were carrying the fighters, their blue curtains drawn, surrounded by military forces.
State news agency SANA said rebels were surrendering more heavy and medium weaponry in Deraa on Sunday.
The so-called "reconciliation" deal for Deraa city is the latest in a string of such agreements the regime has used to retake large parts of the country.
They usually follow ferocious military campaigns and sometimes stifling sieges that effectively force rebels to surrender.
Meanwhile, the Syrian Arab Red Crescent said that a humanitarian aid convoy reached government-held parts of southwest Syria.
Sixteen trucks carrying 3,000 food parcels reached the towns of Nassib and Um Al Mayathen in Deraa province near a border crossing with Jordan, the Red Crescent said in a statement.
The convoy was accompanied by a delegation including the UN humanitarian coordinator in Syria Ali Al Zaatari and representatives of the International Committee of the Red Cross.
On June 19, after securing Damascus and outlying areas of the capital, regime forces — backed by Russia — launched an offensive against Deraa.
The government is now in control of more than four-fifth of that province’s territory, the monitor said.
Rebels and other groups still control 70 per cent of Quneitra, while the Syrian government is in full control of Sweida — a third southern province — the Observatory added.
Quneitra province borders the Israeli-annexed Golan Heights.
Israel has been on high alert in recent weeks as Mr Assad's forces have tackled rebel-held areas of southern Syria.
Mr Abdel Rahman said that unlike in Deraa, fighter planes had not been used in the Quneitra offensive and that Russia was not yet taking part.
The Israeli army said on Friday it had fired a missile and "very probably" destroyed a drone flying over the demilitarised zone between Israel and Syria, after intercepting another drone on Wednesday.
The Assad regime now controls 61 per cent of the country's territory, according to the Observatory.
More than 350,000 people have been killed since the eruption of the Syrian conflict in March 2011.