WASHINGTON - The United States is preparing military options, including surveillance flights, to pressure Islamic State in Syria, US officials said on Monday, but they cautioned no decision had been made to expand US action beyond the limited airstrikes under way 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 militant group, which has increased its overt threats against the United States.

Battling terror must be coordinated with Syrian gov't

General Martin Dempsey, chairman of the Joint Chiefs of Staff, said last week that Islamic State would eventually need to 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, using a different name for the Sunni Muslim group that has seized large areas of Iraq and Syria.

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

A US official said Washington was also preparing to launch intelligence and surveillance flights, including drones, over Syria.

Two other US officials also acknowledged the preparation of strike options against Islamic State in Syria, with one saying planning had been under way for weeks.