The US is now facing a "more abbreviated time frame" to reach a decision on an American response to the escalating situations in Syria and Egypt, US President Barack Obama said on CNN's New Day in an interview that aired on Friday.While UN officials were in Damascus to investigate allegations of chemical weapons use, Obama said he was not optimistic the government of Syrian President Bashar Assad would be cooperative. He warned, however, that the United States' "core national interests" are involved in the Syrian conflict, "both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region."Obama responded to criticism from US Senator John McCain who said that America's credibility in the region has been damaged by the United States' slow response to Syria and Egypt.The president warned against getting "mired in very difficult situations ... (and) being drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region."He also asserted that the US cannot intervene militarily in Syria without support from the UN, and that the US needs to "try to work within an international framework to do everything we can to see Assad ousted.""If the US goes in and attacks another country without a UN mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it," he told CNN's Chris Cuomo.The US Congress is pushing Obama to cut off the $1.2 billion aid the US provides the military-backed Egyptian government, after a military crackdown on Muslim Brotherhood supporters resulted in the deaths of about 900 people.After the violent crackdown began, Obama canceled a biannual military exercise between the US and Egyptian armies, and delayed delivery of fighter jets. The US administration is currently "doing a full evaluation of the US-Egyptian relationship," the president said, and added there was "no doubt that we can't return to business as usual, given what happened."Obama also told CNN that the aid itself "may not reverse what the interim government does" and warned against the US "being seen as aiding and abetting actions that we think run contrary to our values and our ideals."