Fighting between Syrian security forces and rebels killed at least 15 people on Thursday as Arab leaders gathered at a summit in Baghdad to press Damascus for rapid implementation of a peace plan that President Bashar al-Assad has said he can accept.

Arab leaders, who appear to have backed away from their call on Assad to step aside and hand over to a deputy, remain split over how to deal with the continuing violence.

Pre-empting the summit, Syria said on Wednesday it would reject any initiatives from the Arab League, which suspended Syria in November, and said it would deal only with individual Arab states.

In Istanbul, Syrian opposition representatives met to try to settle deep internal disputes before the arrival of Western foreign ministers for a Friends of Syria conference on Sunday to map out where the year-old uprising is heading.

The opposition Syrian Observatory for Human Rights, which monitors the violence, reported 13 civilians, fighters and soldiers killed in clashes across the country.

In northern Hama province, an army convoy was ambushed and two soldiers killed. In Idlib province three people died when the army raided a rural area east of Maarat al-Nuaman.

In the city of Homs, three people were killed by army fire. Two died when the army opened fire in villages near the border with Lebanon and three were killed in clashes in rural districts of northern Hama province, the Observatory said.

The state news agency SANA said two colonels were shot dead in a morning attack in Aleppo, Syria's second city.

Four terrorists shot Abdul Karim al-Rai and Fuad Shaaban ... while they were on their way to work, SANA said.

U.N. Secretary-General Ban Ki-moon, attending the summit in Baghdad, has said Assad's acceptance of the peace plan proposed by Kofi Annan is an important initial step that could bring an end to the violence.

He urged Assad to put those commitments into immediate effect. Western powers have expressed scepticism about Assad's acceptance of the plan.

U.S. State Department spokeswoman Victoria Nuland said on Wednesday that Assad has not taken the necessary steps to implement the peace plan of Annan, the former U.N. Secretary General who is now special Syria envoy for the U.N. and the Arab League.

Syria's big-power backers, Russia and China, have inched up the pressure on Assad by endorsing the Annan plan, with the unspoken implication that if he fails to act on it, they may be prepared to back action by the U.N. Security Council.

But Russia is also pressing the opposition Syrian National Council formally to accept the Annan proposals, which do not meet their demand that Assad step down immediately.

ARAB VIEWS DIFFER

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

Annan's six-point plan 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.

Diplomats say one of his ideas is for a U.N. observer mission to monitor any eventual ceasefire, a mechanism likely to require a U.N. Security Council mandate. An Arab League mission last year failed to make any difference to the crisis.

The United Nations says Assad's forces have killed 9,000 people. Damascus blames foreign-backed terrorists for the violence and says 3,000 soldiers and police have been killed.

As U.S. Secretary of State Hillary Clinton prepared to visit Saudi Arabia and later Turkey to consult Gulf states and promote unity in Syrian opposition ranks, there was little sign that President Barack Obama's administration was ready to change its hands-off approach.

Unless opposition splits are healed, there is little chance that Assad's opponents can oust him without a military intervention the West clearly does not want.

The Obama administration's approach to the crisis in Syria will continue to be wary and slow-moving, said Michael O'Hanlon, a military expert at the Brookings Institution.

If Assad has reached a turning point and really made headway against insurgents, I believe there is a good chance he will 'win' without too much American pushback, O'Hanlon said.

(Additional reporting by Patrick Markey in Baghdad, Dominic Evans and Oliver Holmes in Beirut and Missy Ryan in Washington; Writing by Douglas Hamilton; Editing by Tim Pearce)