WASHINGTON: The United States is poised to impose sanctions on Turkey over its acquisition last year of Russian S-400 air defense systems, four sources including two US officials told Reuters on Thursday, a move likely to worsen already problematic ties between the two NATO allies.

The long-anticipated step, which is likely to infuriate Ankara and severely complicate relations with the incoming administration of President-elect Joe Biden, could be announced any day, sources said.

The sanctions would target Turkey's Presidency of Defence Industries and its head, Ismail Demir, sources have said. Several sources said they were expecting the announcement on Friday but one said it could come any day and perhaps as soon as Thursday.

The Turkish lira weakened following the news. US sanctions could harm a Turkish economy already struggling with a coronavirus-induced slowdown, double-digit inflation and badly depleted foreign reserves.

The US State Department could alter its plans and widen or narrow the scope of planned sanctions against Turkey, a NATO ally for decades.

However, sources said the announcement of the sanctions in their current form was imminent and suggested the United States -- after long waiting for Turkey to change course in the face of US warnings about sanctions requirements under the law -- was finally ready to act.

The Pentagon referred questions on any potential sanctions to the State Department. The State Department did not immediately respond to a request for comment.

Russia delivered the ground-to-air S-400s last year and Turkey tested them as recently as October. Ankara said they would not be integrated into NATO systems and pose no threat, and has called for a joint working group on the issue.