Russia's military said Wednesday it is adding more anti-aircraft missiles to its defense systems in the Russian-annexed Crimean peninsula.

The move comes as Russia and Ukraine are locked in a dispute over a Sunday incident in which Russian forces seized three Ukrainian vessels and their crews in the Black Sea.

Russia accused the sailors, who were manning three Ukrainian navy ships, of illegally entering Russian waters and ignoring warnings from Russian border guards, charges Ukraine has denied.

On Tuesday, a court in Crimea ordered 12 of the 24 apprehended sailors to be detained for two months. The rest of the group was due to appear in court on Wednesday.

Ukraine has imposed martial law in some of its border regions in response to the incident, and with a growing number of other European countries, has urged Western allies to impose additional sanctions on Moscow.

WATCH: US responds to Russia-Ukraine conflict

President Petro Poroshenko said the martial law will help "strengthen Ukraine's defense capabilities amid increasing aggression and according to international law a cold act of aggression by the Russian Federation." He demanded Russia release the Ukrainian sailors and vessels.

Russian President Vladimir Putin expressed concern about the decision to impose martial law and told German Chancellor Angela Merkel he hoped Germany could influence Ukraine to avoid any "reckless acts."

Putin is due to meet this week with U.S. President Donald Trump as leaders from the G-20 nations gather for a summit in Argentina.

But in an interview with the Washington Post, Trump said, "Maybe I won't have the meeting."

He said the decision would depend on his reading of a full report on the situation he was due to receive late Tuesday, and added, "I don't like that aggression."

Kremlin spokesman Dmitry Peskov told reporters Wednesday that Russia has not received any information about canceling the meeting.