The convoys, each comprising some 20 trucks, crossed into Syria from the Turkish border town of Cilvegozu on Tuesday. However, the trucks carrying mostly food and flour just passed the Turkish customs post and still remain stranded in the border region as of Wednesday.
David Swanson, the spokesman for the United Nations Office for the Coordination of Humanitarian Affairs, said that 20 UN trucks were "ready" to leave the border region.
"Things are taking longer than we'd hoped," he noted, adding that "some groups" were blocking aid getting into the militant-held eastern part of Aleppo.
Video footage posted online on Wednesday showed militants blocking Castello road, a key supply route that the humanitarian aid for the city has to pass through.
Swanson further stressed that the UN was further prepared to deliver relief aid to "other besieged and hard-to-reach locations... but only once access is possible."
The developments come a day after the Syrian government said it would reject any unauthorized aid, adding that the delivery of humanitarian aid to Aleppo, especially those provided by Turkey, should be coordinated with the Damascus government and the UN.
The aid convoys were dispatched after a nationwide ceasefire, brokered by the United States and Russia, went into effect in Syria on Monday.
The seven-day truce is the second attempt this year by Washington and Moscow to bring an end to the Syrian crisis, which started in March 2011.
The UN estimates that well over half a million people are living in besieged areas across Syria.
According to UN Special Envoy for Syria Staffan de Mistura, more than 400,000 people have been killed in the conflict in Syria. The UN has stopped its official casualty count in Syria, citing its inability to verify the figures it receives from various sources.
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