Russia on Wednesday said it would bolster its defenses along the Black Sea by sending another advanced anti-aircraft system to the Crimean peninsula amid heightened tensions after a naval standoff resulted in three Ukrainian ships and their crews being seized.

Col. Vadim Astafyes, a top defense ministry official, was quoted by the Interfax news agency as saying Russia will deliver another S-400 anti-aircraft missile system to the three already deployed on the Russian-controlled peninsula.

The RIA news agency said the missiles could be ready by the end of the year.

While the deployment seems to have been planned for a while, the timing sends a message that Russia intends to defend territory and waters it considers its own.

Developed by Russia in the late 1980s, the military hardware holds four missiles, each capable of hitting a target about 250 miles away and intercepting a plane or rocket as far away as 370 miles.

A Reuters reporter in Crimea also saw a Russian navy minesweeper heading for the Sea of Azov, a body of water that Ukraine uses to access two of its ports.

The developments come after Russian forces fired on and seized three Ukrainian vessels and 24 crew members Sunday in the Black Sea and Kiev imposed 30 days of martial law in response.

Russian leader Vladimir Putin, in his first public comments about the incident, accused Ukrainian President Petro Poroshenko of orchestrating the “provocation” as a political ploy to give his flagging poll ratings a shot in the arm before elections next spring.

“It was without doubt a provocation,” Putin said in Moscow. “It was organized by the president ahead of the elections. The president is in fifth place ratings-wise and therefore had to do something. It was used as a pretext to introduce martial law.”

Russia has accused Ukraine of trying to create an incident to help persuade the US and Europe to add more sanctions on top of the penalties imposed after Russia illegally annexed Crimea from Ukraine in 2014.

The Kremlin has also been backing pro-separatist groups in eastern Ukraine, where fighting has killed more than 10,000 people.

Ukraine said its ships — two gunboats and a tugboat — were abiding by maritime law when they tried to pass through the Kerch Strait on the way to one of their ports on the Azov Sea.

With Post wires