President Petro Poroshenko told the country’s Security Council earlier this month that he did not plan to extend the martial law beyond one month, initially foreseen unless there was a large scale attack from Russia.
24 Ukranian crew members, arrested by the Russians, remain in custody – pending trial.
The Ukrainian navy ships were captured in the Kerch Strait, between Russian-annexed Crimea and southern Russia, which controls access to the Sea of Azov, where there are both Russian and Ukrainian ports.
Moscow said the ships entered Russian waters while trying to cross the strait without prior notice, ignoring orders to stop.
Kiev said its ships needed no Russian permission to cross the strait.
Under the martial law, Ukraine banned Russian men of combatage from entering the country and boosted security at critical sites such as nuclear power stations and Black Sea ports.
Ukraine’s military chief said this month Russia had been ramping up its forces near the border since August and now poses the greatest military threat since 2014, the year it annexed Crimea.