The Syrian opposition has cast doubt on whether UN-backed peace talks would go ahead on 9 March as planned.

Rebel groups claim they were under fierce government attack near the Turkish border despite a cessation of hostilities agreement.

The agreement drawn up by the United States and Russia came into effect on Saturday and has slowed but not entirely stopped the conflict, which is approaching its sixth anniversary.

Both the government and rebels have accused each other of violations.

The agreement does not include Islamic State or al Qaeda's Nusra Front, which is widely deployed in opposition areas.

The US has expressed concerns about reports of government tank and artillery attacks against civilians in several areas, as well as possible use of chemical weapons.

Yesterday, the UN said a new attempt at peace talks would begin on 9 March in Geneva, urging warring sides to ensure the cessation agreement take hold to allow them to come to the table.

Opposition official George Sabra said the dates for a resumption of talks remained "hypothetical" as long as the current truce did not fulfill humanitarian demands including a release of detainees held by the government.

"As long as the truce does not help implement the terms (of a United Nations resolution), all dates for the resumption of negotiations remain hypothetical," Mr Sabra told Arabic news channel Arabiya al-Hadath.

The opposition is pressing for full humanitarian access to rebel-held areas and for detainees to be released - terms set out in a UN Security Council resolution passed in December.

Opposition officials say an increase in aid access has fallen short of what is required.

A senior US official said they were working with Russia to improve access to besieged areas.

The World Health Organisation said it delivered medical supplies to the besieged town of Mouadamiya today, after reporting some medicines had been removed from a previous aid delivery.

There was no immediate comment from the Syrian government.

Yesterday, President Bashar al-Assad said insurgents had breached the deal from day one and the Syrian army was refraining from responding to give a chance for the agreement to last, warning that there "are limits".

Five months of Russian air strikes have turned the momentum Mr Assad's way in the war that has killed more than 250,000 people and created refugee crises in neighbouring states and Europe.

Antony Blinken, deputy U.S. Secretary of State, said in Geneva that major and regional powers were monitoring the cessation of hostilities to "prevent any escalation" but it was a "challenging process".

"The best possible thing that could happen is for the cessation of hostilities to really take root, and to be sustained, for the humanitarian assistance to flow and then for the negotiations to start," he said.

While residents of some parts of Syria are describing an unusually calm spell, rebels say government forces backed by Russian air strikes have continued offensives in areas of strategic importance in northwestern Syria.

The Syrian government is saying very little about military operations in those areas, where the Nusra Front is widely deployed in close proximity to groups fighting under the banner of the Free Syrian Army that have accepted the agreement.