Humanitarian aid to war-stricken citizens in Syria is still being blocked by the government, despite a truce arranged by the United States and Russia.

Barack Obama has voiced his "deep concern" over the situation, warning the terms of the ceasefire were not currently being met.

The White House said the US President held a meeting with national security aides, in which he warned that the US would "not proceed with the next steps in the arrangement with Russia until we see seven continuous days of reduced violence and sustained humanitarian access."

The ceasefire, which has been in place for four days, was reluctantly accepted by rebels who criticised it for being biased towards the Assad regime - but said they had no choice due to the desperate need for aid by the civilians affected by the war.

But the government has yet to deliver letters guaranteeing aid workers access to Aleppo to hand out food and supplies.

Two aid convoys destined for the city remain stuck on the Turkish border while the sides argue over how to deliver the goods to those in need.