Ukraine’s president is demanding that Russia immediately release Ukrainian sailors and ships seized in a standoff in the Black Sea.

President Petro Poroshenko said at a meeting of Ukraine’s National Security Council Monday that “we demand that they are urgently turned over to the Ukrainian side” and called for a “de-escalation” of the crisis around Crimea.

Later on Monday, the Ukrainian parliament is set to consider a presidential request for the introduction of martial law.

Ukraine’s navy said six Ukrainian seamen were wounded when Russian coastguard opened fire on three Ukrainian ships near the Kerch Strait and then seized them late Sunday.

The two nations traded blame over the incident. It aggravated tensions that have soared since Russia annexed Ukraine’s Crimean Peninsula in 2014 and backed a separatist movement in eastern Ukraine.