WASHINGTON — Even as it prods Turkey to step up in the global fight against Islamic State militants, the United States is worried that Ankara might use military action to target Kurdish fighters who are the last line of defense against extremists trying to take over the Syrian border town of Kobani.

In a careful-what-you-wish-for scenario, U.S. officials acknowledge that drawing Ankara into the war could open a new line of attack against a Kurdish movement that has for decades sought greater autonomy inside Turkey.

At the same time, Americans officials fear Turkey could simply choose to remain out of the fray, and let two of its enemies — the Islamic State group and Kurdish guerrillas — fight for Kobani. That would give the militants an opportunity to do as much damage to the Kurdish fighters in Syria as possible.

Neither scenario is agreeable, the officials said. The issues and implications are expected to be broached — delicately — when U.S. envoys coordinating the international response to the Islamic State group meet Thursday and Friday with Turkish leaders in Ankara. The officials spoke on condition of anonymity because they were not authorized to discuss the diplomatic situation by name.

For months, Turkey resisted using force against the Islamic State, which has rampaged through large amounts of territory just over its borders in Iraq and Syria. Until recently, its reluctance had been mostly excused out of security concerns for dozens of Turkish diplomats and employees who were kidnapped by the militants from the Iraqi city Mosul in June.