The U.S. envoy for Syria has urged rebel groups to respect a shaky nationwide truce after they gave its brokers 48 hours to stop a regime offensive near the capital.

Twenty-nine rebel groups on Sunday set the deadline for the U.S. and Russian sponsors of the February 27 ceasefire to halt an offensive by President Bashar Assad's regime in the Damascus region.

The Pentagon on Friday rejected a Russian proposal to launch joint air strikes against jihadists from next week in Syria, where they have been conducting separate bombing campaigns.

"We do not collaborate or coordinate with the Russians on any operations in Syria," Pentagon spokesman Navy Captain Jeff Davis told reporters, explaining that Washington and Moscow have separate military objectives.

The U.S.-led coalition fighting the Islamic State group has dropped flyers on Syria's Raqa urging residents for the first time to leave the jihadist stronghold, an activist and a monitor said Friday.

"It's not the first time that coalition airplanes have dropped flyers on Raqa but it's the first time that they've addressed residents and asked them to leave," a founding member of the citizen journalist group Raqa is Being Slaughtered Silently (RBSS) said.

Syrian troops backed by fighters from the Lebanese Shiite movement Hezbollah retook a key town and nine surrounding villages outside Damascus from rebels on Thursday, a monitor said.

An aid convoy entered a besieged rebel-held suburb of Syria's capital on Wednesday, the International Committee of the Red Cross said, in its first delivery there in four years.

Twenty-nine trucks carried food and medicine for some 10,000 people in Harasta in Eastern Ghouta, a rebel bastion under regime siege since mid-2013, ICRC spokesman Pawel Krzysiek said.

A regime air strike killed at least 13 members of the same family, including eight children, in central Syria on Wednesday, a monitor said.

The raid hit a residential neighborhood in Rastan, one of the last rebel strongholds in the central province of Homs, the Syrian Observatory for Human Rights said.

At least 50 fighters and two civilians were killed Tuesday in clashes between rival anti-regime groups east of Syria's capital, the Syrian Observatory for Human Rights said.

The powerful Jaish al-Islam, or Army of Islam, has been locked in clashes with rival factions led by al-Qaida's Syrian affiliate in the opposition stronghold of Eastern Ghouta.

World and regional powers working to end the Syrian conflict vowed to bolster the country's shaky ceasefire Tuesday but failed to set a new date for peace talks.

U.S. Secretary of State John Kerry and his Russian counterpart Sergei Lavrov put on their now customary joint appearance, but strains in the peace process were evident.

Fighting raging between rival Islamist rebel factions to control a key opposition stronghold near Damascus since late last month has killed more than 300 fighters, a monitor said on Sunday.

The Syrian Observatory for Human Rights said the clashes in Eastern Ghouta pitted the Saudi-backed Jaish al-Islam faction, which has been taking part in peace talks in Geneva, against the Faylaq al-Rahman and Jaish al-Fustat groups, both led by Al-Nusra Front, Syria's Al-Qaida affiliate.

Belgium will extend its F-16 air strikes against Islamic State jihadists in Iraq into Syria, the government said Friday, as it grapples with the aftermath of deadly IS-claimed bomb attacks in Brussels in March.

"In accordance with U.N. Resolution 2249, the engagement will be limited to those areas of Syria under the control of IS and other terrorist groups," a spokesman for Prime Minister Charles Michel told AFP after a cabinet meeting.