Sunday's incident has sparked global condemnation over Russia's use of force.
The conflict pitting Ukrainian soldiers against Russian-backed separatists in the breakaway Donbas region, in eastern Ukraine, has sputtered along for nearly five years with more than 10,000 people killed. A Russian ship rammed a Ukrainian military tugboat before coast guard vessels opened fire on the two Ukrainian gunships it was accompanying.
Guterres underscored "the need to fully respect the rights and obligations of all concerned parties" under worldwide law and said "the sovereignty and territorial integrity of Ukraine, within its internationally recognized borders. must also be fully respected".
One of them parroted the version of events put forward by Russian authorities, saying, "The actions of the Ukrainian armed vessels in the Kerch Strait had a provocative character".
Antonio Guterres said he was "greatly concerned" about the situation and called on "both parties to exercise maximum restraint and to take steps without delay to contain this incident and reduce tensions".
North Atlantic Treaty Organisation said Stoltenberg expressed the US -led military alliance's "full support for Ukraine's territorial integrity and sovereignty, including its full navigational rights in its territorial waters under worldwide law".
Poroshenko said it was necessary because of intelligence about "a highly serious threat of a ground operation against Ukraine".
Poroshenko wrote on Twitter that he declined to impose martial law for 60 days to avoid affecting residential election campaigns. Poroshenko's approval ratings have been plunging, and there were concerns that he would postpone a presidential election scheduled for March. The troops were being relocated to an area near the city of Kerch, following the tense standoff on Sunday with a group of Ukrainian ships that, despite repeated warnings, had breached Russia's territorial waters in the Kerch Strait.
The incident was the first major confrontation at sea in the long-running conflict pitting Ukraine against Moscow and Russian-backed separatists in the country's east.
At an emergency United Nations Security Council session requested by Ukraine, outgoing U.S. Ambassador Nikki Haley condemned Russia's "outlaw actions" in violation of global law, but made no mention of whether the U.S. might toughen existing sanctions against Moscow.
Seized Ukrainian seamen speak on Russian TV, and it is not clear if they are speaking under duress.
A Russian fighter-bomber fired two missiles at Ukrainian ships during Sunday's naval clash, causing serious injuries to an officer aboard one of the vessels, Ukraine's security services said Tuesday.
Russian Federation and Ukraine have blamed each other for Sunday's clash in the Kerch Strait, which links the Black Sea and the Sea of Azov. Donald Trump gave a subdued response to the clash, saying "we do not like what's happening either way".
A vehicle is stopped at a checkpoint manned by Ukrainian police officers, in Berdyansk, south coast of Azov sea, eastern Ukraine, Nov. 27, 2018.
"It's not a political issue here, because we can have an argument about the legal status, but it's about simply concentrating on protecting them and helping them", Foreign Minister Pavlo Klimkin told the AP.
Sunday's incident, Ukrainian officials say, is a direct result of the lack of Western reaction to the unfolding imposition of a de facto sea border, which has been slowly but surely throttling access to the Sea of Azov and the Ukraine's important Mariupol industrial region.
The European Union, Britain, Canada, France, Germany and others expressed support for Kiev on Monday, in statements pro-Kremlin newspaper Izvestia denounced as "predictably anti-Russian". Russia's deputy ambassador to the United Nations, Dmitry Polyansky, told an emergency gathering of the Security Council that the Ukrainian ships had made a "pre-planned" provocation "in full connivance with western states".
His Russian counterpart Sergey Lavrov blamed Kiev for what he described as a "provocation", adding: "Ukraine had undoubtedly hoped to get additional benefits from the situation, expecting the USA and Europe to blindly take the provocateurs' side".