A video of three crew members from the Ukrainian vessels seized by Russia confessing to violating a border has been released by their captors.
Footage was distributed by Russia's main intelligence agency, the FSB, and shows interviews with three seamen.
In the footage, they all admit to having violated the Russian border, an act which prompted the military response.
It is not yet possible to ascertain if the men were talking under duress while one of them appeared to be reading from a teleprompter.
On Sunday, Russian border guards fired at three Ukrainian navy vessels as they attempted to make their way through the Kerch Strait, near Russian-occupied Crimea.
The Russians then seized the ships along with their crews, with the escalation over the weekend becoming the first overt military confrontation between Russia and Ukraine.
After the boats were detained, Ukraine's president Petro Poroshenko introduced martial law, involving a military government and the suspension of ordinary law.
He stated that the 30-day measure, approved by parliament on Monday evening, was to “strengthen Ukraine’s defence” against a “pretty unpredictable neighbour”.
It has never before been imposed in Ukraine.
Russia and Ukraine blamed each other in the latest dispute.
Ukraine said its vessels were heading to the Sea of Azov in line with international maritime rules.
However, Russia stated they had failed to obtain permission to pass through the narrow Kerch Strait, spanned by a bridge that Russia completed this year.
This has further escalated tensions since Moscow annexed Crimea in 2014, throwing its weight behind separatists in eastern Ukraine with clandestine support.
Previously, a dramatic recording suggested a Russian military ship was ordered to “squash” a Ukrainian boat in a maritime clash.
The footage appeared to show one of the country's ships deliberately smashing into the much smaller Ukrainian vessel, while a man can be heard shouting: “Squash him, well done, squeeze him from the right...”
According to Ukrainian sources, six of its naval crew were wounded while 23 were detained following Russian fire on the boats.
Russia is said to have then seized two armoured ships and a tug.
Russia has alleged Ukraine orchestrated the incident in a bid to boost the poll ratings of President Poroshenko in coming elections.
The state security service of the Ukraine said its intelligence officers were among the crew on the ships.
The SBU agency, in a statement on Tuesday, said officers were fulfilling counterintelligence operations for the Ukrainian navy, in response to "psychological and physical pressure" by Russian spy services.
Russia's FSB intelligence agency also said there were SBU officers on board the Ukrainian ships, which it described as proof of a "provocation" staged by Ukraine.
Both sides have been urged to de-escalate the conflict by Western leaders and diplomats.
While the US blamed Russia for what it called "unlawful conduct".
This page is being updated.
