ISTANBUL • In the clearest sign of his pivot towards Russia and away from Nato and the West, Turkish President Recep Tayyip Erdogan announced that Turkey has signed a deal to purchase a Russian surface-to-air missile system.

The deal cements a recent rapprochement with Russia, despite differences over the war in Syria, and comes as Turkey's ties with the United States and the European Union have become strained.

It is certain to stir unease in Washington and Brussels, where officials are trying to keep Turkey - a long- time Nato member, and an increasingly unlikely candidate for EU membership - from entering Russia's sphere of influence.

Although a prospective missile purchase from Russia was made public several months ago, Mr Erdogan's announcement on Tuesday was the first confirmation that Turkey had transferred money to pay for the missile system, known as the S-400. The purchase flies in the face of cooperation within the Nato alliance, which Turkey has belonged to since the early 1950s.

Nato does not ban purchases of military hardware from manufacturers outside the US-led alliance, but it does discourage members from buying equipment not compatible with that used by other members.

A Nato official in Brussels, the headquarters of the alliance, said no member currently operates the Russian missile system, and that the alliance had not been informed about the details of the purchase.

"What matters for Nato is that the equipment allies acquire is able to operate together," the official said. "Inter-operability of allied armed forces is essential to Nato for the conduct of our operations."

NYTIMES