Russia has announced it will deploy new S-400 defense missile systems on the Crimean Peninsula amid a standoff initiated by Ukraine.

Russia’s Defense Ministry said on Wednesday that it would deploy new S-400 surface-to-air missile systems on the Crimean Peninsula.

On Sunday, Russia’s naval forces intercepted and seized three Ukrainian vessels after they illegally entered Russian waters off the coast of Crimea in the Sea of Azov.

The Kremlin has said the seizure of the ships was lawful because they were trespassing.

Still, that development set off an international dispute in which Kiev and its Western allies have accused Moscow of trying to assert dominion in the Sea of Azov.

Meanwhile, Ukraine has announced the application of martial law for 30 days in parts of the country following the seizure of the three vessels.

Russian President Vladimir Putin has voiced “serious concern” over Ukraine’s introduction of martial law, conveying the concern in a phone call to German Chancellor Angela Merkel, the Kremlin announced in a Tuesday statement.

The martial law gives Ukrainian officials the power to mobilize citizens with military experience, control the media, and impose restrictions on public rallies. That has in turn raised speculations that the tensions may spiral into an armed conflict.

In Ukraine's east, the military has already been cracking down on ethnic Russians since 2014.

The RIA news agency reported that the new S-400 missile systems would be operational by the end of the year. Footage released on Wednesday showed the new missiles participating in drills at the Kapustin Yar test range in the Astrakhan region.

According to the Russian Defense Ministry, the S-400 systems practiced destroying low-flying, high-speed, highly maneuverable targets, relocation after completing combat missions, countering diversion groups, and moving through contaminated areas.