Russia delivered the system last year to Ankara and Turkey tested the system in October -- in the face of Washington's repeated condemnations and warnings over punitive measures.

US President Donald Trump, who once called Erdogan a "good friend", avoided sanctioning Turkey under a 2017 law known as CAATSA, which lays out sanctions against nations that buy significant quantities of arms from US adversaries including Russia.

But with weeks left of his presidency, the Washington Post reported Thursday on Trump's plans to impose sanctions against Ankara "in the coming days for purchasing and testing" the S-400s.

"While they (US administrations) say with pride 'We have a NATO country like Turkey', for them to now stand up and confront Turkey with CAATSA, once more it's a disrespect to a very important NATO partner," Erdogan said.

"I don't know where this will lead to before Trump leaves but during the four-year Trump period, I didn't have any problems in communicating with America," Erdogan told Turkish journalists as he returned from a visit to Azerbaijan.

Ties between the US and Turkey have been strained over multiple issues, including US support for a Kurdish militia in Syria viewed as terrorists by Ankara as well as the S-400s.

But Erdogan appeared to suggest relations could improve under Biden, as he said he knew the former vice president under Trump's predecessor Barack Obama.

"He is someone who knows me very well. And I know him very well," said Erdogan, who has been president since 2014 and prime minister before that from 2003.

While experts say Biden will be more engaged in multilateral diplomacy than Trump, they also warn he could introduce democracy and human rights issues into relations.

The US threat comes as European Union leaders agreed Friday to draw up a list of Turkish targets for sanctions, in response to Turkish energy exploration in the eastern Mediterranean Sea's disputed waters.

The embattled Turkish lira declined further Friday to reach 8.02 against the US dollar, before rallying to 7.96 towards 1000 GMT, a loss of over one percent on the day.

The lira has lost around 25 percent in value against the greenback since the start of 2020.