A total of 135 people were killed in the first week of a partial truce in Syria in areas covered by the deal, a monitoring group, highlighting its fragile nature just days before the United Nations attempts to reconvene peace talks.

U.N. envoy Staffan de Mistura said the talks, originally due to begin on Monday in Geneva, would get off to a staggered start later in the week, with delegates arriving from Wednesday onwards.

The U.N. said the delay was due to "logistical and technical reasons and also for the ceasefire to better settle down".

"I see us beginning on (Thursday) March 10 when we will launch the process," de Mistura said in an interview with pan-Arab newspaper Al Hayat.

A pro-Damascus Lebanese TV channel, al-Mayadeen, reported from its own sources that talks had been moved to March 13. Reuters could not independently verify this.

The five-year Syrian civil war has killed more than a quarter of a million people and created a massive refugee crisis in Lebanon, Turkey and the European Union.

The partial truce, drawn up by Washington and Moscow, came into force a week ago and has slowed the pace of the war, although it does not include Islamic State militants or the al Qaeda-linked Nusra Front.

The opposition is dissatisfied with the implementation of the deal and has yet to say whether it will attend the new talks. Fighting continues in many parts of Syria, and rebels say the Syrian government, backed by Russian air power and fighters from Iranian-backed Hezbollah, has kept up attacks on strategically important frontlines.

Fighting has also continued between rebel groups and Kurdish-backed forces in north Aleppo, and between rebel groups and Islamic State.

On Saturday Islamic State regained control of a border crossing with Iraq seized by a group of rebels on Friday, the Syrian Observatory for Human Rights said.

The Observatory said 135 people have been killed in areas covered by the 'cessation of hostilities' agreement since it came into force on Feb. 27. In areas not covered by the truce, 552 people were killed, said the Britain-based group, which tracks the conflict via sources on the ground.

Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry, in a phone conversation late on Friday, called for a prompt start to the peace negotiations, Russia's Foreign Ministry said in a statement.

"The two sides called to start the negotiations as soon as possible...between the Syrian government and the whole spectrum of the opposition, during which the Syrians themselves should determine the future of their country," the ministry said.

ASSAD'S DEPARTURE

Saudi Foreign Minister Adel al-Jubeir, whose country backs the rebels, said on Saturday that Syrian President Bashar al-Assad must leave at the beginning of a political transition, not at the end.

"For us it is very clear it's at the beginning of the process, not at the end of the process, it's not going to be 18 months," Jubeir said during a visit to France.

Assad, however, enjoys firm backing from Iran and Russia and his military position has strengthened, especially since Russia entered the war by launching waves of air strikes last September. The United States and other Western governments that previously called for the president's early departure have quietly backed away from that demand.

De Mistura attempted to convene peace talks in January, but these failed before they had even started in earnest. The new talks will be conducted indirectly, not face-to-face, he told Al Hayat.

The fall-off in violence has made aid deliveries easier in some areas of the country, but de Mistura said the Syrian government should be processing aid faster.

"Lorries are waiting for 36 hours," he said. "And medical aid must be allowed."

On Wednesday the World Health Organization said Syrian officials had rejected the delivery of medical supplies, including trauma and burn kits and antibiotics, in a convoy to the besieged town of Moadamiya two days earlier.

Syrian opposition coordinator Riad Hijab said on Friday conditions for talks were "not favourable" and medical and food supplies were being blocked despite the truce.

On Saturday, the opposition Syrian National Coalition said it had named a new president.

Anas Abda will replace Khaled Khoja as head of the group, the SNC said in a statement on its Twitter account. – Reuters