Russia fired on two Ukrainian naval ships and rammed a third vessel in the Black Sea Sunday, seizing the ships, claiming they illegally entered its territorial waters.
Ukrainian officials say at least six sailors were wounded and denies doing anything wrong - accusing Russia of engaging in military aggression.
"Such actions pose a threat to the security of all states in the Black Sea region and therefore require a clear response from the international community," Ukraine's Foreign Ministry said.
The Russian Federal Security Service (FSB) accuses Ukraine of staging a deliberate provocation.
Video gleaned from Russian social media purportedly showing a Russian vessel ramming a Ukrainian ship:
"Weapons were used with the aim of forcibly stopping the Ukrainian warships. As a result, all three Ukrainian naval vessels were seized in the Russian Federation's territorial waters in the Black Sea," an FSB statement said.
Sunday's incident began when a Ukrainian tugboat set out to escort two navy ships from Odessa, on the Black Sea, through the Kerch Strait to the Ukrainian port of Mariupol, in the Sea of Azov.
The Kerch Strait is the only passage between the two seas.
A 2003 treaty designates the Sea of Azov and the pathway into it - the Kerch Straight - as shared territorial waters.
Ukraine says a Russian Coast Guard ship rammed and damaged the tugboat and fired at the two other ships. Russian Coast Guardsmen boarded all three vessels and took them over.
Ukraine says Russia used a tanker to block access to the Kerch Strait.
Russia says the Ukrainian ships were violating its waters and accuses the Ukrainians of failing to inform it that three of its ships were planning to sail through Kerch - a charge Ukraine denies.
Ukrainian President Petro Poroshenko called an emergency meeting of his war cabinet Sunday night. Poroshenko says he will propose that parliament declare martial law in the wake of the attack.
An emergency United Nations Security Council meeting has been called for tomorrow at 11:00 a.m., U.S. envoy to the U.N., Nikki Haley, tweeted late Sunday.
Both NATO and the European Union issued statements late Sunday urging both sides to act with the "utmost restraint" while NATO calls on Russia to "ensure unhindered access to Ukrainian ports in the Azov Sea in accordance with international law."
The Trump administration has warned Russia against trying to strangle the Ukrainian economy by harassing international shipping through the Kerch Strait.
Russia annexed Ukraine's Crimean Peninsula in 2014, claiming its ethnic Russian majority there was under threat from the Ukrainian government.
Meanwhile, fighting between pro-Russian separatists in eastern Ukraine and Ukrainian troops has eased in recent months, but there are still occasional deadly flare-ups.
Russia has consistently denied sending weapons and fighters to the help the rebels, despite strong evidence to the contrary.