Russia deployed another S-400 air defense missile system on Thursday in northern Crimea amid tensions in the Kerch Strait, the authorities said.

The fourth battalion of the newest S-400 system took up combat duty in northern Crimea, Russian Black Sea Fleet spokesman Alexei Rulev said.

The other three battalions have been deployed in the eastern, western and southern parts of the peninsula since Crimea was incorporated into Russia in March 2014 following a local referendum.

Before the deployment in Crimea, the fourth battalion successfully completed drills at the Kapustin Yar testing ground in Astrakhan Region, according to the Russian Defense Ministry.

Russia has strengthened its forces in Crimea amid ongoing confrontation with Ukraine in the Kerch Strait and the Sea of Azov.

Ukraine put its armed forces on full combat alert and imposed 30 days of martial law after Russian forces shot at and seized three Ukrainian naval ships near the Kerch Strait for allegedly violating the Russian border Sunday.

Russia's S-400 system is designed to destroy air targets, including warplanes, ballistic and cruise missiles, at a distance of up to 400 km and at an altitude of up to 30 km.