ANKARA — Turkey's military began launching strikes on Kurdish insurgency bases in northern Iraq late last month in an offensive it has yet to announce publicly.

After two years of threats, Ankara has sent ground forces, attack helicopters and fixed-wing aircraft to Kandil mountain strongholds of the Kurdish Workers Party. Most of the weapons used by Turkey were imported from the United States, Middle East Newsline reported.

The government of Prime Minister Recep Erdogan has not reported the attacks. But Kurdish sources in southeastern Turkey said Ankara began attacks in late April against PKK strongholds with F-16 multi-role fighters, AH-1G attack helicopters and M-60 main battle tanks.

The Turkish military said in a statement this week that PKK insurgents were infiltrating Turkey from Iraq in increasing numbers. The military said the insurgents were bringing large amounts of explosives into Turkey. [On Tuesday, four Turkish soldiers were killed in a landmine explosion.]

The main target of the Turkish military has been the PKK base in Kandil, about 330 kilometers north of Baghdad. Kurdish sources said the Turkish strikes were brief, did not contain a large number of troops and deemed as in hot pursuit of PKK fighters fleeing into Iraq.

Over the weekend, Turkish troops killed nine PKK operatives, including a Syrian national, in the Tunceli province in southeast Turkey. Later, three Turkish soldiers were reported killed. About 10,000 Turkish soldiers participated in the operation In Washington, the Bush administration has warned Turkey's military against crossing into Iraq.

"We are very aware of the Turkish concerns about the presence of the PKK, which we and they both see as a terrorist organization in northern Iraq," State Department spokesman Richard Boucher said on May 11. "We are very committed to try to ensure that the territory of Iraq is not used for terrorist actions or as a terrorist safe haven and throughout Iraq we're taking action to try to carry through on that commitment."