DAMASCUS  - International envoy Kofi Annan urged Syria's Bashar al-Assad to immediately implement a ceasefire, as fighting raged on Friday even after the embattled president said he had accepted the peace plan.

Thousands of demonstrators took to the streets across Syria to protest against what they regard as the inaction of Arab governments in the face of a crackdown that the UN says has cost more than 9,000 lives since March 2011.

UN-Arab League peacebroker Annan's ceasefire appeal came as monitors said shells rained down on Homs, a main rebel bastion which has been the focus of much of President Assad's year-long crackdown on anti-regime protests.

"We expect him to implement this plan immediately. Clearly we have not seen a cessation of hostilities on the ground. This is our great concern," the spokesman said, adding the "deadline is now" for Assad's regime to end all violence.

The plan calls for a commitment to stop all armed violence, a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate, and release of arbitrarily detained people.

"I can't tell you what the next steps will be if they don't stop now," the spokesman said, adding however that Annan was due to brief the UN Security Council on Monday and "we will take it from there."

Annan is also working to convince the Syrian opposition to "lay down their arms and start talking," he said.

State-run news agency SANA said on Thursday that "President Assad... has informed Annan that Syria approves the plan (the envoy) submitted but had made remarks about it."

Assad would "spare no effort" for the success of Annan's six-point plan but said the proposal would only work if "terrorist acts" by foreign powers stopped.

The Britain-based Syrian Observatory for Human Rights said at least 24 civilians and a soldier were killed in violence on Friday.

Protesters took to the streets despite a fierce assault by security forces on the town of Maaret al-Numan in Idlib province in the northwest.

"The desertion of the Arabs and the silence of the Muslims are the hardest things facing Syrians," read a sign held up at a protest by hundreds of people in the Idlib settlement of Kafaroma.

Meanwhile, the United States hit Syria's defence minister and two other senior military figures with sanctions Friday, ramping pressure on Bashar al-Assad's regime as UN efforts failed to halt violence.  The Treasury Department announced measures against Dawoud Rajiha as well as the army's deputy chief of staff and the head of presidential security.

In a sternly worded statement the Treasury Department said it would hold individuals accountable for a bloody crackdown that has claimed an estimated 9,000 lives and injured thousands more.

"The United States will continue to work with our partners around the world to ensure that the Syrian regime and its senior officials are held accountable for the continued repression in Syria."