The first UN convoy carrying aid from Turkey into Syria was halted on Friday, less than 24 hours after the long-awaited humanitarian mission began, with one aid official blaming "administrative hurdles" thrown up by the Syrian government.
The convoy only became possible last month after the UN Security Council reached a rare moment of unity on Syria, calling on all sides to provide humanitarian access.
Eight out of a total of 79 trucks carrying medicine, food and bedding to civilians in the Kurdish city of Qamishli were able to cross before the delay, as co-ordinators sought permission from the Syrian government to clear customs during the Syrian weekend, the United Nations said.
"We have overall agreement from the Syrian government but today it's the weekend and the Syrian part of the border is closed," UN regional humanitarian coordinator Nigel Fisher said.
"Thursday was Mother's Day - a public holiday in Syria so the government sent a special letter to customs to open the border and we are working to see if the same would be possible on Saturday but movement on Friday is very unlikely," he said.
Aid organisations have until now resorted to expensive and limited air drops to deliver aid, but agencies including WFP, UNHCR, and WHO are hoping the convoy can bring vital supplies to more than 50,000 people.
The blockage is a reminder of the difficulties reaching the estimated 9.3 million people in Syria in need of help.
One western aid official accused the Syrian government of hampering efforts to deliver aid.
"The regime will use all sorts of hurdles to delay things. The Security Council resolution calls for free and unhindered access. With these administrative hurdles it is not free and unhindered," he said, asking not to be identified.
He also questioned the UN's strategy of seeking daily permission to get the aid moving.
"It may have been better to apply for blanket access ... What we need are continuous pipelines and this does not address that."
Western officials and Syrian residents have also expressed concern that those in rebel-held areas won't benefit from the supplies, due to be distributed by partners of the Syrian government.
Syrian authorities were not immediately available for comment.
UN Secretary-General Ban Ki Moon is expected to report to the Security Council within 30 days of aid starting to flow, and "further steps in the case of non-compliance," could be considered, according to last month's resolution.
However diplomats say it is unlikely that Russia or China would support sanctions against Damascus.