Turkish Prime Minister Recep Tayyip Erdogan met Saudi King Abdullah in Riyadh Friday for talks on the Syrian conflict, amid Gulf Arab calls to arm rebels despite a tenuous truce going into effect.

"The major issues in the region, particularly those concerning the Syrian question, are on the agenda," Turkish ambassador Ahmet Muhtar Gun said.

"Turkey attaches huge importance to coordination with Saudi Arabia, its most important partner in the region," he added.

Turkey, which shares a 910-kilometre (560-mile) border with Syria, has severed its former alliance with Damascus over the regime's brutal 13-month crackdown on dissent, and has emerged as the main haven for Syrian opposition groups and rebel fighters.

The two leaders were also expected to "examine options in the event that (peace envoy Kofi) Annan's plan fails," the Saudi head of the Gulf Research Centre, Abdel Aziz al-Saqr, said.

Among the alternatives being considered if the touted 0300 GMT Thursday ceasefire fails was "assistance for the Syrian opposition to defend itself, including the possibility of arms supplies," Saqr said.

Annan's plan calls for the withdrawal of forces from urban areas, a halt to fighting, a daily two-hour humanitarian truce, the release of arbitrarily detained people, freedom of movement for journalists and the right to demonstrate.

Saudi Arabia, with Qatar, has been the leading champion of arming rebel fighters in their battle to overthrow President Bashar al-Assad's regime.

The two leaders were also expected to discuss the "impact of any violation by Syrian armed forces of the border with Turkey."

Saudi Defence Minister Prince Salman bin Abdul Aziz held talks at the Pentagon on the crisis in Syria on Wednesday hours before the ceasefire deadline was due to expire.

"Clearly both countries share a concern about what's happening in that country," Pentagon press secretary George Little told reporters after the meeting between Prince Salman and US Defence Secretary Leon Panetta.

The Saudis have called for arming rebels fighting the regime in Syria but the United States has been willing to provide only "non-military" assistance to the opposition in the form of radio equipment.