Ukraine's president declared martial law and North Atlantic Treaty Organisation leaders gathered for emergency talks Monday, a day after a naval clash with Russian warships sparked one of the most serious military threats in the region in recent years.
The Ukrainian parliament approved the introduction of martial law after Poroshenko reassured some sceptical lawmakers that it would not be used to curb civil liberties or delay elections scheduled for next year.
The vote came upon a request from President Petro Poroshenko, who sought to "ensure the state's sovereignty and independence of Ukraine", a declaration on his website said.
"[It's] an arrogant act that the worldwide community must condemn", she added.
Activists in eastern Ukraine, backed by Russian operatives, took over towns and tore down Ukrainian flags. Dmitry Polyanskiy, Russia's first deputy permanent representative to the United Nations, told the Security Council the incident was another example of Ukrainian leaders trying to provoke Russia for political purposes.
U.S. Ambassador to the United Nations Nikki Haley called Russia's actions an "outrageous violation".
U.S. Secretary of State Mike Pompeo called Russia's seizure of the Ukrainian vessels "a unsafe escalation and a violation of global law" and called for restraint from both countries. Video of the incident released by Ukraine shows a Russian ship deliberately ramming into a Ukrainian tugboat.
"Russia rams Ukrainian vessel peacefully travelling toward a Ukrainian port. Russian Federation seizes ships and crew and then accuses Ukraine of provocation???"
"This is a very risky provocation, which requires particular attention and a special investigation", Russian presidential spokesman Dmitry Peskov told state media.
Russian seizure of Ukrainian naval ships off Crimea sparks alarm
"Russia must release detained Ukrainian sailors and guarantee free passage through Kerch Straits".
In May, President Vladimir Putin personally opened the bridge over the Kerch Strait, connecting the Crimea peninsula - which Moscow seized in 2014 - to Russia's mainland.
A senior fellow at the pro-NATO Atlantic Council has said the United States should send naval ships into the Sea of Azov to guarantee it stays open after a skirmish between Russian and Ukrainian ships.
In another sign of rising tensions, Russia's state-controlled RIA news agency reported on Sunday night that Ukrainian forces had started heavy shelling of residential areas in eastern Ukraine, which is controlled by pro-Moscow separatists. At the moment, they are heading at full speed in the direction of the Kerch Strait to the zone where provocation was organized by the Ukrainian side.
Transit under the bridge has been blocked by a tanker ship, and dozens of cargo ships awaiting passage are stuck. Russia, Ukraine's closest neighbor and major trading partner, vehemently opposed the deal, fearing an uncontrolled flow of goods through what was then virtually an open border.
"Having breached this morning the Russian border, the three Ukrainian ships - the Berdyansk, the Nikopol and the Yani Kapu - tried again on November 25 at 19:00 (Moscow time) to conduct illegal actions in Russian territorial waters", the FSB said on Sunday.
Sporadic violence grew into a full-blown conflict in May 2014, when Ukraine launched an airstrike on the Donetsk airport that was overrun by Russian Chechen fighters. He reportedly said he called the meeting, "to discuss unsafe Ukraine provocations in the sea of Azov and Kerch Strait near Crimea threatening global peace and security".
German Chancellor Angela Merkel also spoke by telephone with Mr Poroshenko to express her concerns and emphasise the need for de-escalation and dialogue, her office said.