Amid renewed tension in eastern Ukraine, Russia has sent 15 warships to the Black Sea for a manoeuvre, which passed through the Kerch Strait on the Crimean Peninsula on Saturday, the Navy said.
It was not initially said how long the exercises would last, dpa news agency reported.
The US had previously cancelled the deployment of two warships to the Black Sea following complaints from Russia.
On Friday, the Russian Defence Ministry declared that certain sea areas in Crimea would be closed off for months because of the manoeuvres.
However, shipping in the strait was not affected, it said.
The European Union, NATO and Ukraine criticised Russia's actions.
In response, a senior EU official said it was an "extremely worrying development".
International concern is growing about an escalation of the conflict in eastern Ukraine because of Russian troop deployments not far from the Ukrainian border.
Meanwhile, the head of Ukraine's military operation, Sergei Nayev, indicated that he does not expect a new war.
"We don't see any attack preparations there," he told Ukrainian television on Friday night.
"In fact, field hospitals have been set up." All units are in camps, on firing ranges, he said.
"In military language, we are talking about a show of force."
Meanwhile, a Ukrainian diplomat has been arrested in the Russian city of St Petersburg.
He had obtained classified documents, the domestic intelligence service FSB announced, which is not compatible with the status of a diplomat.
Russia, Ukraine expel diplomats over classified information
Russia has ordered a Ukrainian diplomat to leave the country after allegedly receiving classified information from a database of the country's main security agency and Ukraine responded by expelling a Russian diplomat.
Alexander Sosonyuk, the Ukrainian consul in St. Petersburg, was detained Friday while meeting with a Russian in which he obtained material from a database of the Federal Security Service, according to the agency.
On Saturday, the Russian foreign ministry informed charge d'affaires Vasily Pokotilo that Sosonyuk must leave the country by Thursday. No details about the contents of the classified material were made public.
The Ukrainian foreign ministry later said it has ordered a senior diplomat to leave, but didn't identify the person.
The expulsions come amid escalating tensions over Russia's military buildup along the border with the eastern Ukraine region that is under the control of Russia-backed rebels.