WASHINGTON -  President Barack Obama warned that the United States should avoid getting mired in costly and difficult foreign inventions as calls mounted Friday for military action against Syria over alleged chemical warfare.

In an interview broadcast Friday, Obama said allegations of a new chemical weapons attack by government forces on Syrian civilians were of “grave concern” But he also pointed out obstacles to US military action, a year after warning that the use of chemical arms in the vicious Syrian conflict would cross a US “red line.”

In an interview with CNN, he said opposition allegations that hundreds of people had been killed in a gas attack near Damascus this week were more serious than previous charges against President Bashar al-Assad’s regime.

Obama told CNN that US authorities were gathering information about the attack, which produced horrifying footage of dead children and victims gasping for air. “What we’ve seen indicates clearly this is a big event, of grave concern,” Obama said. Reports in several US newspapers detailed possible US military options including cruise missile strikes against Syrian targets, which top generals are preparing for Obama’s deliberations. But they also suggested there were divisions in the administration over the consequences of another US intervention in the Middle East.

Obama introduced a note of scepticism despite calls from critics like Republican Senator John McCain for US military strikes. “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 national security 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,” Obama said.

He warned that America could get “drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region,” he said.

The president also said that there were questions whether the United States would infringe international law if it attacked another country without a United Nations Security Council mandate. Syria has vigorously denied its forces were guilty of a chemical attack on the rebel-held area.

Russia on Friday said that mounting calls in Europe and elsewhere for the use of force against Syria were “unacceptable.”

Foreign Minister Sergei Lavrov, in a statement issued after he held a telephone conversation with his US counterpart John Kerry, said Moscow “called on the Syrian government to cooperate with the UN chemical experts”.

Lavrov also appealed for rebels to allow the UN inspectors on the ground in Syria probing three other suspected chemical attack sites safe access to areas where the latest alleged attacks occurred.

Meanwhile, UN chief Ban Ki-moon said Friday that the use of chemical weapons in Syria would constitute a “crime against humanity”, adding there was “no time to lose” in probing alleged attacks which the opposition says killed hundreds.

Ban described reports of the incidents near Damascus on Wednesday as “very alarming and shocking” and urged the regime to allow a United Nations inspection team, already on the ground in Syria, to begin an investigation without delay.

Footage distributed by activists, showing unconscious children, people foaming around the mouth and doctors apparently administering oxygen to help them breathe, has triggered revulsion around the world.

Ban’s comments, at a United Nations event in Seoul, piled more pressure on Syrian President Bashar al-Assad after French President Francois Hollande denounced the “likely” use of chemical weapons.