US President Barack Obama says new allegations of chemical weapons use by Syrian forces are of "grave concern".
But he's cautioned against the United States intervening hastily and getting "mired in very difficult situations" with actions that could "breed more resentment" in the region.
In an interview with CNN broadcast on Friday he said opposition allegations that hundreds of people had been killed in a gas attack near Damascus on Wednesday were more serious than previous charges against President Bashar al-Assad's regime.
"We are right now gathering information about this particular event, but I can say that unlike some of the evidence we were trying to get earlier that led to a UN investigator going into Syria, what we've seen indicates clearly this is a big event, of grave concern," he said.
Obama stopped short of saying point blank the United States believes such weapons were used - an act that in theory would cross what he has called a "red line".
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He reiterated that the United States is seeking to gather more evidence on whether chemical weapons were in fact used this week.
The president has been loath to order US military action to protect civilians in Syria, fearing being drawn into a vicious civil war, just soon after he extracted US troops from Iraq.
But revulsion over video footage and photos of dead children blanketing the US media has reopened the debate about a policy Obama allies see as prudent but critics brand as weak.
Republican Senator John McCain warned on Thursday that Obama had given Assad a "green light" to commit atrocities by failing to use military force to respond to previous chemical attacks.
"When the president of the United States says that if he uses these weapons that it would be a, quote, 'red line and a game-changer,' (Assad) now sees that as a green light," he told CNN.
Obama answered McCain in the new CNN interview, saying he understood the lawmaker's worry but that the United States had to proceed with caution.
"I am sympathetic to Senator McCain's passion for helping people work through what is an extraordinarily difficult and heart-breaking situation," he said.
But Obama said Americans expect him to protect their long term interests.
"Sometimes what we've seen is folks will call for immediate action, jumping into stuff that does not turn out well, gets us mired in very difficult situations, can result in us being drawn in to very expensive, difficult, costly interventions that actually breed more resentment in the region," he said.
