Barack Obama has said the US has "no strategy" against the Islamic State in Syria but said officials had been asked to prepare a range of military options for confronting the group.

The US president's comments on Thursday cautioned against speculation that he was close to a decision to expand his air campaign against the Islamic State beyond Iraq.

"I don't want to put the cart before the horse," Obama said. "We don't have a strategy yet. I think what I've seen in some of the news reports suggests that folks are getting a little further ahead of where we're at than we currently are."

"They have no ideology beyond violence and chaos and the slaughter of innocent people," Obama said, adding that Middle Eastern countries must drop their ambivalence towards the group.

He said his top priority remained rolling back the group's gains in Iraq, where he has said they pose a threat to US personnel in Erbil and Baghdad.

"Our focus right now is to protect American personnel on the ground in Iraq, to protect our embassy, to protect our consulates, to make sure that critical infrastructure that could adversely affect our personnel is protected,'' he said.

Obama has come under increasing pressure to deal with the Islamic State in Syria after months of territorial gains, and the murder of the US journalist James Foley just over a week ago.

US officials earlier this week stated that it was flying spying missions over Syria to gather intelligence on the group.

Obama's comments came after activists posted the latest evidence of Islamic State atrocities in Syria, with pictures showing the aftermath of the group's takeover of a Syrian army base in Raqqa province.

The armed group shot some and stabbed others using knives in the latest brutal mass killing, activists said on Thursday.

The UK-based Syrian Observatory for Human Rights said the fighters rounded up the soldiers near Tabqa airfield, three days after seizing the base in heavy fighting.

Rami Abdurrahman, the observatory's director, said the fighters killed up to 65 soldiers. He said some were shot to death, while others were killed with knives.

A statement posted online and circulated on Twitter by supporters of the Islamic State group claimed the fighters killed "about 200" government prisoners captured near Tabqa.

The photos could not immediately be verified, but correspond to other reporting by the AP news agency.

Meanwhile, reports on Thursday said that the Islamic State had "waterboarded" hostages in Syria, including the American journalist James Foley.

Waterboarding, a simulated drowning technique, was used by the US on captives in the wake of the attacks of September 11, 2001.