WASHINGTON (Reuters) - President Barack Obama on Friday defended the U.S. approach toward the civil war in Syria, saying he understood the desire for action to end the conflict but it would have been impossible to do “on the cheap” without a full U.S. military intervention.

“Unless we were all in and willing to take over Syria, we were going to have problems,” Obama told a news conference, noting that it would have required “putting large numbers of U.S. troops on the ground, uninvited, without any international law mandate.”

Obama’s remarks came as efforts to evacuate civilians from the last opposition-held areas of the Syrian city of Aleppo ground to a halt on Friday after weeks of bombardments by the Syrian army that have deprived insurgents of nearly all their territory in the city, a divided zone during the nearly six-year civil war.

A Syrian official source said the evacuation was halted because rebels had sought to take out people they had abducted with them, and they had also tried to take weapons hidden in bags. This was denied by Aleppo-based rebel groups.

Obama denounced claims from the regime of Syrian President Bashar al-Assad and its allies that all innocent civilians who were trapped in Aleppo had been able to leave, saying the reports were efforts to “obfuscate the truth”.

“Humanitarian organizations who know better and who are on the ground have said unequivocally that there are still tens of thousands that are trapped and are prepared to leave,” Obama said at Friday’s news conference. “So right now our biggest priority is to get them out.”

Obama also addressed President-elect Donald Trump’s vow at a rally in Pennsylvania on Thursday to “get the Gulf states to give us lots of money” to help “build safe zones in Syria”. He said any safe zones would need to be maintained by on-the-ground forces unless Trump can secure the cooperation of the Assad regime and its allies.

The United States will continue pressing the United Nations Security Council to help improve the delivery of humanitarian aid to civilians in the besieged city, and to monitor for any potential use of chemical weapons in Syria, Obama said.

“Responsibility for this brutality lies in one place alone: with the Assad regime and its allies, Russia and Iran,” he said. “This blood, and these atrocities, are on their hands.”