Turkey is assessing whether to delay taking delivery of the S-400 Russian missile defence system, currently scheduled for July, after a new request from the United States, a source familiar with the matter said on Monday.

Axar.az reports citing Euronews that, the move would be a rare pause in months of escalating tensions between the two NATO allies, which have been at loggerheads over Turkey's planned purchase of the Russian system, which the United States opposes.

Washington requested last week that Ankara consider postponing taking delivery of the system, a source familiar with the matter said on the condition of anonymity.

There was no immediate response from Turkey, which has said repeatedly it was not going to back down from its planned purchase. Last week, Fahrettin Altun, spokesman for Turkish President Tayyip Erdogan, said the procurement of S-400s was a 'done deal.'

The source said talks on the issue continued.

U.S. officials have called Turkey's planned purchase of the S-400 missile defence system "deeply problematic," saying it would risk Ankara's partnership in the joint strike fighter F-35 programme because it would compromise the jets, made by Lockheed Martin Corp.

The United States and other NATO allies that own F-35s fear the radar on the Russian S-400 missile system will learn how to spot and track the jet, making it less able to evade Russian weapons.

Ankara has also been pushing Washington to establish a working group to assess the risks the system would be posing to the F-35 jet. The United States has so far been reluctant to accept.

The disagreement is the latest in a series of diplomatic disputes between the United States and Turkey. They include Turkish demands that Washington extradite cleric Fethullah Gulen, differences over Middle East policy and the war in Syria, and sanctions on Iran.