Turkey began testing on Monday its Russian missile defence system, local media reported, despite repeated calls from the United States that it could lead to sanctions. Planes, including F-16 fighter jets, circled the Murted military base in Ankara province to test the newly acquired S-400 system and train Turkish operators, news agency DHA reported. Turkey's acquisition of the S-400 in July was met with consternation by its NATO allies.

The US argues there is a risk that sensitive technological information could be leaked if it is used alongside Western equipment such as the new F-35 jet. Turkey has ordered 100 F-35s and its defence industry was part of the supply chain for the new jet, until it was kicked off the programme due to the S-400 purchase.

So far, the US has appeared reluctant to impose threatened sanctions on Turkey over the purchase, with officials saying it could be spared if it does not activate the S-400 system - though this option has been rejected by Turkey.