Turkey has launched a series of aggressive airstrikes against Kurdish militants but has yet to turn its firepower on Islamic State in Syria as expected, increasing concerns in Washington about the Ankara government’s intentions.

Three weeks ago, the countries announced a breakthrough agreement to allow the U.S. to use bases in Turkey to launch airstrikes against Islamic State, and the Ankara government said it would join in the bombings. On Wednesday, the Pentagon said U.S. jets now based at Incirlik Air Base in southern Turkey had hit their first targets in Syria, which American officials cheered as a significant step forward.

The use of the base has strategic value for the U.S., making it easier to hit Islamic State targets just across the border in northern Syria. But some U.S. officials suspect Turkey is using its recent agreement with the U.S. to fight Islamic State as cover for a new offensive against Kurdish separatist group PKK.

A senior U.S. official said Turkey gave American officials assurances last week that it planned to wrap up attacks on the Kurds in short order, but it has kept up the bombardments focused on the group’s bases in northern Iraq near the Turkish border.

“It’s clear that ISIL was a hook,” said a senior U.S. military official, referring to Islamic State. “Turkey wanted to move against the PKK, but it needed a hook.”

The PKK is branded a terrorist organization by the U.S. and Turkey. But its affiliate in Syria has proved an effective ally to the U.S. by battling Islamic State on the ground.