UN Secretary-General Ban Ki-moon met Arab leaders in Baghdad on Thursday to discuss a UN-backed peace plan for Syria as President Bashar al-Assad's forces continued their offensive against cities and towns across Syria.

Regional leaders in Iraq for the Arab League summit have endorsed special envoy Kofi Annan's six-point plan that calls for a ceasefire and peace talks, but they remain sharply split over how to deal with the violence that risks deepening sectarian divisions.

Syria has accepted Annan's proposal, but rejects any initiatives taken by the summit and said it would deal only with individual Arab states, complicating efforts to implement the UN-backed peace plan on the ground.

"This is an important initial step that could bring an end to the violence and the bloodshed," Ban said of Syria's accepting Annan's plan, before arriving in Baghdad on Thursday.

"I strongly urge President Assad of Syria to put those commitments into immediate effect."

Arab states, while rejecting any foreign intervention in Syria, appear to have backed away from their initial proposal that Assad step aside to allow his deputy to organise talks.

Sunni powers Saudi Arabia and Qatar have led the push to isolate Syria, including suggestions for arming Syria's opposition, but non-Gulf Arab states such as Algeria and Shiite-led Iraq urge more caution, fearing that toppling Assad could spark sectarian violence.

Annan's proposal calls for the withdrawal of heavy weapons and troops from population centres, humanitarian assistance, the release of prisoners and free movement and access for journalists. But it does not explicitly call for Assad to step down from office.

Diplomats say one of Annan's ideas is for a UN observer mission to monitor any eventual ceasefire, a mechanism likely to require a UN Security Council mandate.

An Arab League observation mission in Syria last year failed to end Assad's crackdown on protests, and faced internal dissent. But the League has also discussed a joint UN-Arab mission for Syria.

Even as Arab foreign ministers held talks in Baghdad on Wednesday before Thursday's summit, Syrian forces bombarded cities and towns in southern and northern Syria, forcing thousands to flee violence.

The United Nations says around 9,000 people have been killed in fighting. Damascus blames foreign-backed terrorists for the violence, saying 3,000 soldiers and police have been killed.

The Arab Summit in Baghdad is the first in Iraq for more than two decades and the first hosted by a Shiite Arab leader, Prime Minister Nuri al-Maliki.

Pulling back from years of war, Iraq hopes to use the summit as a way to highlight its return to the diplomatic stage and has sought a fledging detente with Sunni Gulf Arab nations long wary of Iraq's close ties to Shiite power Iran.