"It is deeply regrettable that no patients or accompanying family members could be moved," said Stephen O'Brien, the UN's humanitarian aid coordinator.
"The evacuations were obstructed by various factors, including delays in receiving the necessary approvals from local authorities in eastern Aleppo," he said.
Armed groups imposed conditions to guarantee the safety of the evacuation operation while the Syrian government refused to allow medical and other aid supplies into the rebel-held east of the city, O'Brien added.
Russia, Syria's main ally in the war against opposition rebels and jihadists, last week declared a humanitarian pause in the assault on eastern Aleppo, where more than 250,000 civilians have been trapped.
"After three days of lull, parties to the conflict have still not agreed, military operations have resumed and violence is now escalating," said O'Brien in a statement.
"The political and military paths are trumping basic humanity once again in Syria."
On Monday, Russia ruled out a renewal of the ceasefire and blamed rebel groups for blocking the medical evacuation.
"Over the last three days, what was needed did not happen," said Russian Deputy Foreign Minister Sergei Ryabkov.
In order to renew the pause, "our opponents must ensure appropriate behavior by the anti-government groups that in particular sabotaged the medical evacuation that was intended during the humanitarian pause," he said.
UN Secretary General Ban Ki-moon last week said nearly 500 people had been killed and about 2,000 injured during the nearly month-long bombing raids in Aleppo.
No UN aid convoy has entered Aleppo since July 7 and food rations will run out by the end of October, he warned.
More than 300,000 people have been killed in Syria and more than half of the country's population displaced since the conflict began in March 2011 with anti-government protests.
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"The evacuations were obstructed by various factors, including delays in receiving the necessary approvals from local authorities in eastern Aleppo," he said.
Armed groups imposed conditions to guarantee the safety of the evacuation operation while the Syrian government refused to allow medical and other aid supplies into the rebel-held east of the city, O'Brien added.
Russia, Syria's main ally in the war against opposition rebels and jihadists, last week declared a humanitarian pause in the assault on eastern Aleppo, where more than 250,000 civilians have been trapped.
"After three days of lull, parties to the conflict have still not agreed, military operations have resumed and violence is now escalating," said O'Brien in a statement.
"The political and military paths are trumping basic humanity once again in Syria."
On Monday, Russia ruled out a renewal of the ceasefire and blamed rebel groups for blocking the medical evacuation.
"Over the last three days, what was needed did not happen," said Russian Deputy Foreign Minister Sergei Ryabkov.
In order to renew the pause, "our opponents must ensure appropriate behavior by the anti-government groups that in particular sabotaged the medical evacuation that was intended during the humanitarian pause," he said.
UN Secretary General Ban Ki-moon last week said nearly 500 people had been killed and about 2,000 injured during the nearly month-long bombing raids in Aleppo.
No UN aid convoy has entered Aleppo since July 7 and food rations will run out by the end of October, he warned.
More than 300,000 people have been killed in Syria and more than half of the country's population displaced since the conflict began in March 2011 with anti-government protests.
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