Staffan de Mistura told reporters in Geneva that key government ally Russia agreed crucial time was being lost on the deployment of aid convoys, which is a key part of a ceasefire that began Monday.
"The government, I repeat the government, was expected to provide facilitation letters", clearing desperately needed aid to reach several embattled areas.
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"Those facilitation letters, i.e. (the) final permission for the UN to actually reach those areas has not been received," de Mistura said.
"This is something that requires to take place immediately."
He said the truce agreed during marathon talks last week between Russia and the United States was holding "by and large".
"We cannot let days of this reduction of violence to be wasted by not moving forward", on humanitarian aid, he added.