ISTANBUL — In the face of increasing international pressure, Turkey took decisive military action on Monday — not against the Islamic State militants that Turkey’s Western allies have urged it to fight, but rather against the Kurdish militant group that has been battling the Islamic State.

Turkish warplanes struck positions of the Kurdistan Workers’ Party, known as the P.K.K., in southeastern Turkey late Monday. The group, long an enemy of the Turkish state, had put down its weapons last year to talk peace. But on Tuesday, Turkish officials said the Kurdish militants had attacked a military outpost, leading to the government’s first airstrikes against the group in nearly two years.

The action immediately reverberated well beyond Turkey’s borders, because it is an offshoot of the P.K.K. that is struggling to defend the Syrian Kurdish city of Kobani, which has been besieged by Islamic State for weeks.

As that battle has played out under the watch of news organizations, their cameras just across the border in Turkey, it has become a flash point for international criticism of Turkey’s unwillingness to take a bigger role in fighting Islamic State, also known as ISIS or ISIL.