"The most modern air defence systems S-400 were put on standby alert in the Leningrad region to protect military, administrative and civilian sites," the defence ministry said.

The S-400 surface-to-air missile is one of Russia's most advanced weapons.

A Saint Petersburg-based defence ministry official told AFP the systems replaced the previous S-300s but declined to say how many were placed there.

The missiles were first tested at a range in the southern region of Astrakhan before being transferred 2,000 kilometres (1,200 miles) to the north.

The S-400 system is also expected to be deployed this month in Russia's European exclave of Kaliningrad.

NATO member Turkey is planning to buy the S-400s from Russia despite warnings from the transatlantic military bloc it is incompatible with its own air defence systems.