Turkey is to sign a new contract for S-400 missile-defense systems with Russia, according to Moscow, which would risk further souring ties with the US and could trigger threatened sanctions.

Ankara received the first batch of Russia's surface-to-air missile systems systems in July, prompting the US to kick Turkey off its F-35 fighter jet programme.

The Trump administration warned Ankara it could face sanctions, but said Ankara could be spared if it does not activate the S-400 system.

But the Nato member yesterday came one step closer as it began testing a newly acquired Russian missile defence system against American-made fighter jets. Footage of the first trial run, which began on Monday and continued into Tuesday, showed the system’s radars rotating over an air base outside the capital.

US-made F-16 fighter jets were used as mock targets during the exercise, which will likely irk Washington.

Initial tests may just be to ensure that the radars are working properly or to see if they can adequately distinguish between friendly and enemy aircraft.

The US, as well as Nato, had been concerned that the Turkish military might conduct exactly these tests, potentially giving Russia insights into Western jets' capabilities.