WASHINGTON - The United States is preparing military options to pressure the Islamic State in Syria, the U.S. military said on Monday, but officials cautioned that no decision had been made to expand U.S. action beyond the limited air strikes underway in Iraq.

President Barack Obama has so far sought a limited military campaign in Iraq focused on protecting American diplomats and civilians under direct threat. Still, officials have not ruled out escalating military action against the Islamic State, which has increased its overt threats against the United States.

General Martin Dempsey, chairman of the Joint Chiefs of Staff, said last week that the Islamic State would need to eventually be addressed on “both sides of what is essentially at this point a non-existent border” between Syria and Iraq.

Dempsey’s spokesman confirmed on Monday that options against Islamic State were under review and stressed the need to form “a coalition of capable regional and European partners.”

“With Central Command, (Dempsey) is preparing options to address ISIS both in Iraq and Syria with a variety of military tools including airstrikes,” Colonel Ed Thomas said.

“The bottom line is that our forces are well postured to partner with regional allies against ISIS.”

Two other U.S. officials also acknowledged the preparation of strike options against the Islamic State in Syria, with one saying planning had been underway for weeks. Still, neither official suggested U.S. military action there was imminent.

“We’re just not there yet,” said a senior U.S. defense official, speaking on condition of the anonymity.

Republicans on Sunday called for more aggressive U.S. action to defeat Islamic State militants in Syria and Iraq, accusing President Barack Obama of policies that have failed to thwart potential new threats on U.S. soil.

Dempsey, Thomas said, believed that the Islamic State needed to be pressured in Iraq and Syria and that defeating the group would require a sustained effort over an extended period of time “and much more than military action.”

Although the U.S. air campaign launched this month has caused some setbacks for Islamic State, they do not address the deeper problem of sectarian warfare which the group has fueled with its attacks on Shi’ites.

In retaliation for the airstrikes, Islamic State released a video showing one of its black-clad fighters beheading U.S. journalist James Foley.

Western powers fear the IS "caliphate" - a successor state to historic Muslim empires - could become a launchpad for a new round of global terror attacks.

Those fears were exacerbated by the grisly IS beheading of American journalist James Foley who was abducted in Syria. Washington has ramped up its rhetoric following the beheading, calling it "a terrorist attack against our country" and said operations against the group in Syria may also be necessary.

US air strikes have allowed the peshmerga to claw back some lost territory, but the campaign has so far been limited to northern Iraq.

Citizens from various western countries are fighting with IS, further raising fears that they could carry out attacks at home.

In a statement Sunday claiming a string of attacks in the northern Iraqi city of Kirkuk that killed 24 people the previous day, IS identified two of the three suicide bombers as German.

On Monday, the peshmerga retook three villages in the Jalawla area northeast of Baghdad from militant militants and also held off two assaults elsewhere, officials said. The Kurdish fighters also took control of a main road used by militants to transport fighters and supplies, peshmerga members said.

In Syria, the militants on Sunday won a bloody battle for the Tabqa military airport, the last stronghold of the Damascus regime in the northern province of Raqa, a monitoring group and state media said.

The Syrian Observatory for Human Rights said 346 militants and almost 200 Syrian troops were killed in the six-day battle for the airport.

The victory gives the IS militants full control of Raqa, the heartland of their "caliphate".

On Monday, a suicide bomber attacked worshippers during prayers in eastern Baghdad, killing 11 people, security and medical officials said.

The attack also wounded 32 people, the officials said. It comes three days after suspected Shia militiamen gunned down 70 Sunni worshippers at a mosque northeast of the capital.

Premier-designate Haidar al-Abadi said there is no place for weapons or armed groups outside Iraqi state control, in the wake of the Sunni mosque attack. "I confirm that weapons must remain in the hand of the state - there is no place for any armed group," he told a news conference on Monday.