GENEVA/UNITED NATIONS: There will be no new round of Syria talks for at least two or three weeks, the office of UN special envoy for Syria Staffan de Mistura said in a statement on Thursday, after he consulted the UN Security Council for about two and a half hours.

"He briefed on his intention to start the next round of talks as soon as feasible but certainly not within the next two/three weeks," said the statement.

It said de Mistura wanted to see progress on the ground, particularly relating to the cessation of hostilities and humanitarian access.

"Meanwhile, the special envoy will maintain close and continuous contact with the Syrian parties as well as the members of the ISSG before determining the 'appropriate time' to reconvene the parties to Geneva."

The ISSG, or International Syria Support Group, is the group of countries led by the United States and Russia that is backing de Mistura's peace efforts. It also includes regional powers such as Saudi Arabia and Iran, who are expected to press the warring parties to respect the statements made by the Security Council.

But Syria's cessation of hostilities, a partial truce brokered by the United States and Russia in February, has been unravelling for weeks, and the Syrian government has largely blocked humanitarian aid access despite repeated UN appeals to prevent civilians starving in besieged towns.

Earlier on Thursday de Mistura had said he expected to announce a date for a new round of talks after consulting the Security Council, citing the need to keep up momentum.

Speaking to reporters in New York after de Mistura's briefing, US Ambassador to the United Nations Samantha Power said the UN mediator was increasingly impatient.

"He expressed his continued frustration with insufficient humanitarian access, the serious dangers to the cessation of hostilities, and the need for real progress on the political talks," she told reporters.

Power said the main threat to the cessation of hostilities was the Syrian government and its allies - and their attacks on civilians.

"Russia has a special responsibility to press the Assad regime to abide by the cessation of the hostilities and end its bombardment and siege of innocent civilians," she said, referring to Syrian President Bashar al-Assad.

Power acknowledged there were concerns about the fact that the US-Russian partnership on Syria had not yet ended the five-year-old war.

"We think the answer is to continue to press Russia and Iran to use the influence they have" on Assad's government, she said.