Ukraine’s security service said it had intelligence officers on the ships seized by Russia during a naval standoff near the Crimean Peninsula as Moscow warned of an escalation in tensions after Kiev imposed martial law.

The head of Ukraine’s security service – the SBU – said in a statement on Tuesday that the officers were conducting counterintelligence operations for the Ukrainian navy in response to “psychological and physical pressure” by Russian spies and demanded they halt such activity.

The statement did not elaborate on what those measures were.

Russia’s intelligence agency FSB acknowledged late Monday that the Ukrainian officers were on the ship, citing their presence as a “provocation.”

Twelve of the 24 Ukrainian sailors detained by Russia were taken to court in Crimea to face charges that they illegally entered Russian territorial waters.

A lawyer for the crew members, Adam Semedlyaev, said they would be tried in three groups – the first one Tuesday and another next week, ABC News reported, citing Hromadske, a Ukrainian television station.

The third group consists of the six men wounded by the Russian forces. They remain hospitalized.

Russian President Vladimir Putin informed German Chancellor Angela Merkel that he is “seriously concerned” about the growing tensions between the two countries after the weekend incident and Ukraine’s instituting martial law.

The Kremlin released a statement after the call.

“The imposition of martial law in some regions may pose a risk of escalating tensions in the conflict-hit region, namely the south-east,” spokesman Dmitry Peskov said on Tuesday.

Ukrainian forces have been battling Russian-backed separatists in the eastern part of the country since Russia illegally annexed Crimea in 2014.

The Black Sea naval standoff has brought worldwide condemnation against Moscow from the US, its European allies and NATO, who have also called for Russia and Ukraine to work toward de-escalating the already high tensions and suggested they would help work out a solution between the two counties.

Russian Foreign Minister Sergey Lavrov, addressing the phone call between Putin and Merkel during a meeting in Paris with his French counterpart Jean-Yves Le Drian, had a message for those asking for a mediator to help resolve the dispute – butt out.

“I do not see the need for any mediators. I think that this is an absolutely practical matter,” he Lavrov said.

Moscow and Kiev continued to trade blame after Russian forces fired on and seized three Ukrainian vessels in the Kerch Strait, which connects the Black Sea with the Sea of Azov, and captured 24 crew members.

In retaliation, President Petro Poroschenko’s government imposed martial law for 30 days in areas of Ukraine near the Russian border and the along coasts of the bodies of water.

With Post wires