BEIRUT: Clashes broke out in Damascus on Thursday as Syrian government forces fired mortar rounds on rebel positions, activists said, in what they described as some of the heaviest fighting in the capital this month.

The violence coincided with the departure from the country of United Nations monitors, whose mandate expired and was not renewed due to deteriorating conditions that have dragged Syria into civil war.

The head of the U.N. mission, Babacar Gaye, was expected to leave on Thursday. Some monitors left on Wednesday and more are expected to follow the team leader on Thursday.

Government forces fired mortar rounds from the Qasioun mountain overlooking Damascus, activists said, and southern suburbs echoed with the sound of gunfights between the army and rebels who have been struggling to topple President Bashar al-Assad for more than 17 months.

"For about an hour we heard explosions and gunfire. It is not as bad as yesterday yet but tensions are really high. I think you could probably hear the echo of shelling or clashes from most parts of Damascus," said opposition activist Samir al-Shami, who spoke to Reuters by Skype.

He and other activists said the bombardment and fighting seemed to be concentrated on southern districts of Damascus, where rebels have been able to slip in from rebellious suburbs near the capital.

Assad's forces stormed the south-eastern district of Kafr Souseh early on Thursday and were making arrests, another activist said.

Hundreds of residents have been displaced due to fighting in the city, and many have taken shelter in mosques and schools in calmer areas. But some of those displaced are being asked to leave their places of refuge, said an activist who gave his name only as Tareq, of the Revolution Leadership Council of Damascus.

"Assad's regime warned people staying in schools they have to evacuate within a few days, they are saying it is because they have to prepare for the coming school year," he said.

"Thousands of displacees are worried because they have a lack of alternatives for a place to stay." - AGENCIES