This marked a new phase in an emerging third-maritime-front between Ukraine and Russian Federation, one likely to keep Europe busy for years to come.
On Sunday, Russia opened fire on three Ukrainian ships and seized their crews in the Kerch Strait between the Black Sea and the Sea of Azov.
They had been traveling from the Ukrainian port of Odessa, on the Black Sea, to the Ukrainian port of Mariupol on the Sea of Azov.
News of missile deployments to Crimea come just a couple of days after a serious naval clash between Russian Federation and Ukraine on Sunday in the Sea of Azov, which is shared territorial waters under a 2003 treaty signed by the two countries.
Russia announces new S-400 surface-to-air missiles deployment in Crimea | November 2018 Global Defense Security
U.S. Secretary of State Mike Pompeo called Russia's seizure of the Ukrainian vessels "a risky escalation and a violation of worldwide law" and called for restraint from both countries.
"We do not like what's happening, either way, we don't like what's happening and hopefully it will get straightened out", he said, hours after U.S. Ambassador to the United Nations Nikki Haley condemned Russia for their "outrageous violation of sovereign Ukrainian territory" that marked "yet another reckless Russian escalation".
France and Germany, which brokered a ceasefire and tentative peace accord between Vladimir Putin, the Russian president, and Petro Poroshenko, the leader of Ukraine, in 2015, are understood to fear such a move could split the bloc and further inflame tensions. The Ukrainian vessels were trying to pass through the Kerch Strait that separates Russian-controlled Crimea from Russia.
He also appealed for the quick release of the captured sailors, who've been given custodial sentences, saying they had clearly been forced to make confessions and should be treated as prisoners of war.
"It is undoubtedly a provocation", the Russian president said, adding that it had been organised by Ukraine's authorities "and, I think, the incumbent president in the run-up to the Ukrainian presidential election in March 2019".
On Monday, Nato chief Jens Stoltenberg called on Russian Federation to free the Ukrainian ships and sailors and said Moscow should realise the "consequences of its actions". Recent polls suggest that only about 10% of the electorate plans to vote for him next year, with and almost 50% saying they would not vote for him under any circumstances, the Kyiv Post newspaper reported.
"Military vessels intruded into Russian territorial waters and did not answer (the border guards)".
Western governments have backed the Ukrainian government.
The statement was strongly worded but will disappoint some European Union members hoping for a harder joint line, after officials had earlier said the bloc was considering new sanctions against Moscow.
The Kremlin said it still expects a meeting between President Vladimir Putin and US President Donald Trump to go ahead as planned despite a suggestion from Trump that it could be cancelled.
Poroshenko called on Germany, the largest and wealthiest buyer of Moscow's gas exports, to halt the building of an undersea gas pipeline that would allow Russian Federation to supply Germany directly, cutting out Ukraine.