Istanbul, Turkey (CNN) -- Turkish warplanes launched a series of cross-border air raids against suspected Kurdish separatist fighters in northern Iraq on Thursday, Turkey's semi-official Anatolian News Agency reported.

A Turkish military source told CNN Turk, CNN's sister network, that the military scrambled jets to carry out the attack after a group of suspected fighters were detected on Iraqi territory, approaching Turkey's mountainous border with Iraq.

Residents of the southeastern Turkish city of Diyarbakir, where a civilian airport doubles as an airbase for the Turkish Air Force, told CNN they heard the unmistakable roar of Turkish F-16 warplanes taking off Thursday.

Meanwhile, the Firat News Agency, widely believed to be a media arm of the Kurdistan Workers Party, or PKK, announced on its website that Turkish warplanes bombed three mountainous regions in northern Iraq on Thursday afternoon.

The Turkish security forces have been battling PKK guerrilla fighters in the predominantly Kurdish region of southeastern Turkey for more than 25 years. More than 30,000 people, mostly ethnic Kurds, have been killed in the conflict.

Kurdish fighters appear to have recently stepped up their attacks on Turkish security forces, as often happens during the summer months.

Since April 26, the Turkish military said, 12 of its soldiers have been killed in a series of attacks in the Turkish southeast. Troops are currently conducting operations in a number of locations in this turbulent region.

For the past several years, Washington, which officially labels the PKK a terrorist agency, has shared "real-time intelligence" with its Turkish NATO allies about PKK activities in the mountains of northern Iraq.

Turkey has long pressured the Iraqi government to do more to rein in Kurdish separatists that operate in the border area.

On May 7, the Turkish military said, it bombed suspected PKK targets in northern Iraq, after a unit of Turkish special forces troops came under fire from across the frontier.