Ukrainian President Petro Poroshenko on Wednesday signed an act introducing martial law after Russia’s seizure of three of Kiev’s navy vessels sparked the worst crisis in years between the neighbours.

‘President Poroshenko has signed the law,’ spokesman Svyatoslav Tsegolko wrote on Facebook, reports france24.com.

Ukraine’s parliament on Monday voted in favour of the president’s request for the introduction of martial law in border areas for 30 days.

There was some confusion over when exactly martial law entered force, however, with some officials saying that it had already begun on Monday. The presidential spokesman was not immediately reachable for comment.

Poroshenko called for the move after Russian forces fired on, boarded and captured three of Kiev’s ships on Sunday off the coast of Crimea.

He warned on Tuesday that Ukraine risked ‘full-scale war’ with Russia and the country’s armed forces have been put in a state of full combat readiness.

Prime Minister Volodymyr Groysman told parliament on Wednesday that ‘we must always be ready to counter the aggression of our enemy, which not long ago was our neighbour’.

Martial law is being adopted in 10 regions that adjoin either Russia’s borders or areas controlled by Moscow-backed separatists, as well as along the coast of the Sea of Azov and the Black Sea.

The prospect of martial law has already led to the relocation of a football match scheduled for Thursday between Arsenal and Ukrainian side FC Vorskla to Kiev from the city of Poltava, even though Poltava is not in one of the affected regions.

The Ukrainian authorities have said that martial law — which allows the mobilisation of citizens, control of media and restricts public demonstrations — is essentially a preventive measure.

Poroshenko in a television interview on Tuesday said that ‘the aim of this martial law is to show that the enemy will pay very dearly if they decide to attack us and that will be like a cold shower that will stop the madmen who have plans to attack Ukraine.’

Ukraine’s defence ministry was set to give a press conference on Wednesday afternoon to explain how martial law will be applied in the armed forces.