Tensions between Ukraine and Russia escalated dangerously on Monday as an emergency UN Security Council meeting was held and the two nations exchanged a volley of threats after conflagrations in the Black Sea on Sunday resulted in Russia seizing Ukrainian vessels by force and left outside observers warning that simmering disputes—mixed with unintended consequences and political instability in Ukraine—could spark wider war between the former allied nations.

The Russian foreign ministry accused Kiev of coordinating with the U.S. and the E.U. in a “planned provocation” that took place in the Kerch Strait, a body of water that separates the Black Sea from the smaller Sea of Azov, which hugs the contested Crimean peninsula.

In the wake of the Russian Navy seizing three vessels and protests in the capitol of Kiev, Ukrainian President Petro Poroshenko on Monday called for the Ukrainian parliament to approve his request to impose martial law as a way of restoring order in the country where both anti-Russian protests have broken out alongside new reports of fighting in the country’s eastern region along the Russian border.

“I will suggest the parliament impose martial law for a term of 30 days. Why? In order to see to it that martial law does not overlap with the election campaign,” Poroshenko said in a televised address to the nation, referring to upcoming elections.

In a tweet on Monday, German Foreign Minister Heiko Maas urged both sides to de-escalate the tensions, calling the recent developments in Ukraine “worrying.”

During the emergency meeting of the Security Council, US Ambassador to the UN Nikki Haley described Russia’s behavior as “outrageous violations” and condemned what she said were “outlaw actions” by the Kremlin.