Statements by three captured Ukrainians have been released by Russia's security service after Russian ships fired on and seized three Ukrainian boats off the coast of Russian-annexed Crimea.
One of the men, Volodymyr Lisovyi, said he was aware of the "provocative nature" of the Ukrainian action.
Ukraine's navy commander said the men had been forced to lie under duress.
Meanwhile, a Crimean court ordered that 12 of the 24 Ukrainians seized on Sunday be detained for 60 days.
The court is expected to issue rulings for the remaining servicemen on Wednesday.
Western nations have condemned Russia's use of force, four years after it seized Ukraine's southern Crimean peninsula.
Why this crisis is fraught with risk
The flare-up is the first outright clash between Ukraine and Russian forces for years, although pro-Russian separatists and Russian military "volunteers" have been fighting Ukraine's army in two eastern regions.
Tensions escalated when Russia opened a bridge this year between Russia and Crimea over the Kerch Strait, which leads into the Sea of Azov. Ukraine has two big ports on the northern shore of the Azov sea, and a 2003 treaty allows both countries free access to its waters.
Russia has accused the two Ukrainian gunboats and a tug of violating Russian territorial waters as they sailed through the Kerch Strait. But Ukraine says the incidents happened in areas that are free to shipping.
Why Ukraine is imposing martial law
On Monday night, Ukraine's parliament responded by backing President Petro Poroshenko's decision to impose martial law for a 30-day period from 26 November in 10 border regions.
On Tuesday, President Poroshenko said there was a threat of "full-scale war" with Russia.
"The number of [Russian] tanks at bases located along our border has grown three times," he said.
Five of the 10 regions border Russia while two are adjacent to Moldova's breakaway Trans-Dniester region, where Russian troops are stationed. The other three regions border the Black Sea or Sea of Azov close to Crimea.
Since April 2014, Ukraine's army has been fighting pro-Russian separatists in two eastern regions of Ukraine bordering Russia, Luhansk and Donetsk.
The move to martial law is unprecedented in Ukraine, and gives military authorities the right to ban protests and strikes. Mobilisation of civilians for military service is possible but not inevitable.
What the sailors said
The Ukrainian sailors were captured by Russia on Sunday and at least three were wounded.
Late on Monday night, the FSB security service released videos of three of the men:
- Andriy Drach said he was on the Nikopol gunboat with an order to sail from Odessa to Mariupol. "We were warned by the border service of the Russian Federation that we were violating Russian law. They had repeatedly asked us to leave the territorial waters of the Russian Federation," he said
- Serhiy Tsybizov said he was also on the Nikopol
- Volodymyr Lisovyi said he was commander of a military unit and was part of a naval task force. "I deliberately ignored requests via ultra-short-wave band," adding that there were small arms on board as well as machine-guns
The head of the Ukrainian navy, Ihor Voronchenko, told Ukrainian TV that the three men had given false statements under duress.
"I know those sailors from Nikopol. They have always been honest professionals in their jobs, and what they say now is not true," he said.
The head of Ukraine's SBU security service, Vasyl Hrytsak, confirmed Russian reports that members of the service were on board the boats, but added that it was a "routine counter-intelligence mission" of a type that the Russian navy carried out regularly.
How the West has reacted
Several countries have condemned Russia's actions on Monday, and the UN Security Council failed to agree a Russian-proposed agenda, and instead discussed a Ukrainian proposal on the issue.
US Secretary of State Mike Pompeo said the seizure of Ukrainian ships was a "dangerous escalation and a violation of international law". President Donald Trump said: "We do not like what's happening, either way we don't like what's happening and hopefully they'll get straight."
The UK condemned Russia's "destabilising behaviour in the region and its ongoing violation of Ukrainian territorial sovereignty".
Taking a call from German Chancellor Angela Merkel, Russian President Vladimir Putin argued that the Ukrainians had "deliberately ignored the rules of peaceful passage in the territorial sea of the Russian Federation", the Kremlin said.
Mrs Merkel had "stressed the need for de-escalation and dialogue", her spokesman said.