The rewards of up to $5 million (4.4 million euros) could help ease tensions with Ankara which has long urged its Western allies to take a tougher line against the PKK, banned as a terror group not just by Turkey but also the United States and the EU.

Matthew Palmer, a US deputy assistant secretary of state, said the rewards had been authorised for "information leading to the identification or location" of Murat Karayilan, Cemil Bayik and Duran Kalkan.

Information about Karayilan could be worth up to $5 million, concerning Bayik $4 million and Kalkan $3 million, he added in a statement released by the US embassy in Ankara.

- 'Received with caution' -

While the State Department has designated the PKK as a terror group since 1997, Turkey has been hugely unhappy over cooperation in Syria between the United States and the People's Protection Units (YPG), which Ankara considers a branch of the PKK.

Bilateral ties also suffered over the detention in Turkey of American pastor Andrew Brunson, which lasted more than two years.

But Brunson was released in October and both sides appear keen on improving the key relationship between the NATO allies.

"The United States values its counterterrorism cooperation with our NATO ally Turkey," said Palmer, adding that the rewards were being issued as part of the State Department's Rewards for Justice scheme.

President Recep Tayyip Erdogan's spokesman Ibrahim Kalin said Turkey greeted the news "with caution" and described the decision as "late".

"They could not fool us by saying that the YPG was different from the PKK," he told HaberTurk television. "It will soon become clear if this is a cover for the YPG."

After two successful military operations inside Syria, Turkey now has its sights on the area of Manbij which is controlled by the YPG and has a US presence.

On its official Twitter feed, the Rewards for Justice programme posted pictures of the three men under the headline "Reward for Information".

"Provide information and payment may be possible. 100% confidentiality guaranteed. Relocation may be possible," it said.

Karayilan and Bayik are seen as the de facto leaders of the PKK on the ground following the capture by Turkey of its founder and leader Abdullah Ocalan in 1999.

Ocalan is serving a life sentence on a prison island off Istanbul. Karayilan and Bayik are believed by analysts to be in the remote Qandil mountains region of northern Iraq where the PKK has its rear bases, although some experts think it is possible that senior PKK figures also slip on occasion into neighbouring Iran.

The PKK has waged a three-and-a-half decade insurgency against the Turkish state seeking independence, and more recently autonomy, for Turkey's Kurdish minority.

The conflict has left tens of thousands dead and is still continuing after the PKK halted a ceasefire in 2015.