"We are working with our counterparts elsewhere in Europe and in Turkey to address the PKK and work cooperatively, to find and cut off financing to terrorist groups, be it PKK, Al-Qaeda" or others, Mueller told reporters.

"There have been concrete results and there will continue to be concrete results around the world, in Europe and elsewhere," he added.

Mueller spoke after a day of talks with senior Turkish police and national intelligence officials, which he said served to stengthen bilateral ties and enable the two countries to cooperate in facing terrorist threats.

Turkish security forces have been locked in a bitter combat with the PKK since the group, listed as a terrorist organization by the European Union, Turkey and the United States, took up arms in 1984 for self-rule in the country's mainly Kurdish southeast.

The conflict has claimed some 37,000 lives since.

Thousands of militants have found refuge in the mountains of northern Iraq since 1999 and have recently begun infiltrating back into Turkey, stepping up anti-government attacks in the southeast since early this year.

Turkey has long criticised the United States for failing to curb the rebels and even threatened cross border operations into Iraq if the threat is not eliminated.

Washington has been leery of committing itself to military action, arguing instead for means to dry up the group's financial resources.