Rebels fought government forces in Damascus on Monday, in the most violent gun battles the Syrian capital has seen since the start of the year-long revolt against President Bashar al-Assad, opposition activists said.

The clashes near the centre of Assad's power base appeared to be an attempt by rebels, who have been forced out of Homs and Idlib and came under attack in the eastern city of Deir al-Zor on Monday, to show they still pose a serious challenge.

Fighting erupted after midnight in the upmarket al-Mezze district when up to six rebels fired a rocket propelled grenade at the house of an army general, said Rami Abdulrahman, who runs the British-based Syrian Observatory for Human Rights (SOHR).

Quoting local residents, he said the gunmen then took refuge in a building where they battled soldiers, gunfire echoing throughout the neighbourhood. Official Syrian news agency Sana said three rebels and one member of the security forces died.

The armed confrontation came just two days after a double car bombing killed at least 27 people in the heart of the city, in a sign that the capital might be slowly sinking into mayhem.

These clashes were the most violent and the closest to the security force headquarters in Damascus since the outbreak of the Syrian revolution, said SOHR's Rami Abdulrahman.

Video footage showed the top two floors of an unidentified apartment bloc scorched by fire, its walls and stairway pitted with bullet holes and shrapnel.

Reports from Syria cannot be independently verified because the authorities have barred access to rights groups and journalists.

The latest violence coincided with the arrival in Damascus of a team of five experts, sent by the U.N.-Arab League envoy for Syria, Kofi Annan, to discuss proposals to deploy international monitors in Syria.

A separate team of experts from the United Nations and the Organisation for Islamic Cooperation, led by the Syrian government, also started a mission to assess humanitarian needs, a source close to the mission said on Monday.

The group is due to visit areas hit by the uprising including the central city of Homs, scene of a month-long siege and military bombardment in February, and Deraa, where the revolt against Assad erupted a year ago.

EASTERN FORCE

Assad is fighting for the survival of his family dynasty, which has ruled Syria for more than four decades, and has rejected calls from much of the West and the Arab world to stand aside.

His troops have launched crackdowns in recent weeks, winning back much lost ground, but the violence has not abated and analysts warn the uprising could degenerate into civil war, pitting Assad's minority Alawite sect against the Sunnis, who make up 75 percent of the 23-million-strong population.

Witnesses said pro-Assad forces stormed the eastern tribal Sunni Muslim city of Deir al-Zor on Monday to seize areas previously held by the Free Syrian Army - a lightly armed and disparate resistance force led by army defectors.

At least one civilian, named as 60-year-old Adnan Khalifa, died in the assault, residents told Reuters.

I heard the sound of several explosions. They could be tanks firing their guns or rebels using dynamite to try and slow down their advance, Tareq, one of the residents, said, speaking by phone from the city, which lies on the road to Iraq.

SANA reported that 13 civilians were shot dead by opposition terrorists near Syria's third largest city Homs on Sunday and said rebels had also destroyed a railway bridge linking Damascus to the southern Deraa region.

Damascus residents told Reuters that the fighting in Mezze included an attack by rebels near the home of Assef Shawkat - Assad's brother-in-law and deputy chief-of-staff of the armed forces. It wasn't immediately clear if he was the prime target.

HUMANITARIAN HOPES

The United Nations says more than 8,000 people have been killed and some 230,000 forced to flee their homes, including at least 30,000 who have escaped abroad. The government says about 2,000 members of the security forces have been killed.

Turkey has raised the prospect of setting up a buffer zone in Syria to protect those trying to flee. Turkey's Disaster and Emergency Management Department said on Monday 279 Syrians had crossed the border between March 18-19, bring the total number of Syrian refugees in the country to 16,446.

The head of the International Committee of the Red Cross, speaking in Moscow where he met Foreign Minister Sergei Lavrov, said the humanitarian situation in Syria was likely get worse, underlining the need for urgent measures to ease the crisis.

The ICRC's Jakob Kellenberger told reporters he hoped Russia would help persuade Assad to allow more access for humanitarian aid to the worst affected areas.

During the meeting, the ICRC received positive indications of support on its operational priorities and its initiative of a two-hour cessation of fighting on a daily basis, ICRC spokesman Hicham Hassan told Reuters in Geneva.

Russia is a long-time ally of Damascus and has also been one of its top arms suppliers.

In a report published on Monday, the Stockholm International Peace Research Institute said arms deliveries to Syria surged 580 percent from 2007 to 2011 compared with the previous five years, with Russia supplying 78 percent of those imports.

(Additional reporting by Dominic Evans in Beirut, Steve Gutterman in Moscow and Stephanie Nebehay in Geneva; Writing by Crispian Balmer; Editing by Giles Elgood/Janet McBride)