Heavy air strikes were reported to have hit rebel-held areas near Damascus as fighting raged across much of western Syria on Friday, hours before a US-Russian plan aimed at halting hostilities was due to take effect.

The "cessation of hostilities" agreement is due to take effect at midnight.

The government has agreed to the plan. The main opposition alliance, which has deep reservations about the terms, has said it is ready for a two-week truce to test the intentions of the government and its Russian and Iranian backers.

The Syrian Observatory for Human Rights monitoring organisation reported at least 26 air raids and artillery shelling targeting the town of Douma in rebel-held Eastern Ghouta near Damascus.

Rescue workers in the opposition-held area said five people were killed in Douma, listing their names on their Twitter feed. Syrian military officials could not immediately be reached for comment.

Damascus has made clear it will continue to target Islamic State and the al Qaeda-linked Nusra Front which are not included in the agreement on the air strikes.

The opposition fears the government will attack rebels on the pretext they are jihadists. The government says the agreement could fail if foreign states supply rebels with weapons or insurgents use the truce to rearm.

Eastern Ghouta is regularly targeted by the Syrian army and its allies. It is a stronghold of the Jaish al-Islam rebel group, which is represented in the main opposition alliance, the High Negotiations Committee, and has been used as a launch pad for rocket and mortar attacks on Damascus.

The HNC groups political and armed opponents of Assad, and many groups fighting in northern and southern Syria had authorised it to negotiate on their behalf.

"We are striving for a real truce," said Fares al-Bayoush, the head of the Fursan al-Haqq rebel group, which fights under the banner of the Free Syrian Army. But he added: "Everyone knows the degree of regime and Russian commitment to agreements."

The Observatory also reported artillery bombardment by government forces and air strikes overnight in Hama province, and artillery bombardment by government forces in Homs province.

Fighting also resumed at dawn between rebels and government forces in the northwestern province of Latakia, where the Syrian army and its allies are trying to take back more territory from insurgents at the border with Turkey.

US President Barack Obama said on Thursday the US was resolved to try to make the deal work but that "there are plenty of reasons for scepticism".