U.S. President Barack Obama said he has asked Defence Secretary Chuck Hagel to prepare a range of options on confronting the ISIS militant group in Syria but said his strategy is still being developed.

Speaking to reporters at the White House Thursday afternoon shortly before meeting with his national security advisors, Obama said his priority is to make sure gains made by ISIS in Iraq are rolled back.

Obama said that because of U.S. airstrikes thus far, ISIS fighters "are losing arms and equipment" and getting pushed back by Iraqi and Kurdish forces.

But as he had pointed out in the past, ISIS's defeat will require an Iraqi inclusive government in its "broader, comprehensive strategy."

"Rooting out a cancer like [ISIS] will not be quick and will not be easy,” he said.

Obama said he has also asked Secretary of State John Kerry to travel to the Middle East to build a coalition against the militant group.

Such a coalition will need to include regional players, namely Sunni leaders. He said he is encouraged that surrounding countries recognize the threat ISIS poses.

Moderate Sunnis key to long-term strategy

Despite the U.S. airstrikes in Iraq, ISIS militants have been able to occupy parts of neighbouring Syria — a political and military quagmire for Obama considering U.S. opposition to the Assad regime.

Obama, however, said he isn't worried about potential U.S. action inadvertently helping Assad because ISIS-occupied areas aren't controlled by the Syrian regime and Assad doesn't seem to have the "capability or reach" to get into those areas anyway. Obama cautioned against speculation that he was on the brink of a decision to expand airstrikes.

U.S. President Barack Obama said defeating ISIS militants will require a long-term strategy that involves an inclusive Iraqi government. (Charles Dharapak/Associated Press)

"I don't want to put the cart before the horse. 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," he said.

Obama did note that degrading the militant group over the long-term requires moderate Sunnis who are able to govern — a choice other than ISIS or Assad for Syrians.

His comments came after ISIS fighters executed scores of Syrian soldiers captured when the militants seized an airbase in the province of Raqqa on the weekend, according to a video posted on YouTube on Thursday.

The video, confirmed as genuine by an ISIS fighter, showed the bodies of dozens of men lying face down wearing nothing but their underwear. They were stretched out in a line that appeared to be dozens of metres long.

A separate pile of bodies was shown nearby. Reuters could not independently verify the authenticity of the video.

"The 250 shabeeha taken captive by the Islamic State [ISIS] from Tabqa in Raqqa have been executed," read the caption posted with the video, referring to the soldiers by the name to forces loyal to President Bashar al-Assad by Islamist militants fighting him.

An ISIS fighter in Raqqa told Reuters via the Internet: "Yes, we have executed them all."

'How many have you killed?'

Islamic State in Iraq and Syria, a radical offshoot of al Qaeda, stormed Tabqa air base on Sunday after days of clashes with the army and said it had captured and killed soldiers and officers in one of the fiercest confrontations yet between the two sides.

ISIS is reported to have executed 250 Syrian soldiers after taking over Tabqa air base on Aug. 24. (Reuters)

The capture of Tabqa, the Syrian army's last foothold in that area, and apparent killing of large numbers of its soldiers shows Islamic State's grip on the north of the country. The group has also seized territory in eastern Syria and large areas of neighbouring Iraq in recent months.

The Syrian Observatory for Human Rights, which monitors the conflict using sources on the ground, said the soldiers who were executed had been trying to escape from the airport when they were captured by militant fighters of the Islamic State.

Who's your father? Do you know who your father is? You can't possibly know who your father is. You're a bastard. - ISIS interrogator

Another video posted online appeared to show the interrogation of at least one Syrian soldier in front of a group of other captured men in their underwear, as voices off camera shout sectarian insults.

The soldier identifies himself as an officer and says he is from the Alawite sect, an offshoot of Shi'ite Islam, like President Bashar al-Assad and the majority of high-ranking military officers. Islamic State members are Sunni Muslims.

"Who's your father? Do you know who your father is? You can't possibly know who your father is. You're a bastard," the interrogator says, using insults suggesting that Alawites are born out of wedlock.

At one point the soldier briefly looks down at the floor and rubs his eyes, another interrogator throws a metal rod at him, making him flinch and then pay attention to the main interrogator again.

"How many have you killed? How many have you raped?" the interrogator shouts. The soldier replies: "None. I've been stationed here in the airport."

Back to hell

The interrogator asks why the soldier had been fighting on behalf of Assad and did not defect and he replies that he would have just been sent back to the army.

"They would have sent you right back to the army? And we're going to send you right back to hell: by slaughter," the interrogator says, making him chant Islamic State slogans.

Syrian state media confirmed the attack on the base but has not reported any deaths or any army members being captured. It has said Islamic State suffered heavy losses in the battle over the base.

Tabqa was the army's last foothold in an area otherwise controlled by the Islamic State militants, who aim to set up a trans-border caliphate in the Iraqi and Syrian territory they have captured.

The United States has carried out air strikes on Islamic State in Iraq and has left open the option for similar action in Syria.