A boy carries an opposition flag as rebel fighters and civilians gather during the arrival of an aid convoy of Syrian Arab Red Crescent and United Nation (UN) to the rebel held besieged town of Kafr Batna.

Syrian rebels said on Wednesday they were under fierce government attack near the Turkish border despite a cessation of hostilities agreement and a representative cast doubt on whether U.N.-backed peace talks would go ahead on March 9 as planned.

The agreement drawn up by the United States and Russia came into effect on Saturday and has slowed but not entirely stopped a conflict that has been going on for almost five years. Both the government and rebels have accused each other of violations.

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

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

But opposition official George Sabra said the date for a resumption of talks remained “hypothetical” as long as the truce did not fulfil humanitarian demands including a release of detainees held by the government.

“What is the value of a truce if its overseers – meaning America and Russia – do not push all sides to abide by it?” Sabra told Arabic news channel Arabiya al-Hadath on Wednesday.

The White House said it had seen a reduction in air strikes against the opposition and civilians in Syria in recent days but was concerned by some reported tank and artillery attacks.

Washington was also aware of reports of possible chemical weapons use by the Syrian government, the State Department said, adding that it could not confirm them but that they were being investigated. Israel said on Tuesday Syrian forces had been dropping chlorine barrels on civilians over the past few days.

There was no immediate comment from Damascus, which has denied breaching the terms of the truce.

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

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

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