RUSSIA threatened retaliation today after Finland’s leaders said they were in favour of applying to join Nato and with Sweden expected to follow suit within days.

In Moscow, the Foreign Ministry warned that Russia “will be forced to take retaliatory steps of military-technical and other characteristics in order to counter the emerging threats to its national security.”

This statement followed Finnish president Sauli Niinisto and Prime Minister Sanna Marin announcing that the Nordic country should immediately submit an application for membership of the Western military alliance.

“You [Russia] caused this. Look in the mirror,” Mr Niinisto said this week, referring to President Vladimir Putin’s invasion of Ukraine.

While Finland’s parliament has still to have its say, the announcement means Finland is all but certain to apply, though the process could take months to complete. Sweden, too, is considering applying.

Such an expansion of the Western military alliance would leave Russia surrounded by Nato countries in the Baltic Sea and the Arctic.

Nato secretary-general Jens Stoltenberg has said that the alliance would welcome Finland and Sweden with open arms.

Nato supplies of weapons and other military support for Ukraine have already been crucial to Kiev’s ability to resist the Russian invasion, leading the Kremlin to issue a fresh warning today that the aid could lead to an apocalyptic military confrontation between the alliance and Russia.

“There is always a risk of such conflict turning into a full-scale nuclear war, a scenario that will be catastrophic for all,” said Russian security council deputy head Dmitry Medvedev.

On Wednesday, Boris Johnson visited Sweden and Finland and signed security deals with the two countries that commit Britain to aid them, including with military support, if they come under attack.

In Ukraine, Russian forces pounded eastern areas, including in the Azovstal steel mill, the last pocket of resistance in Mariupol, while Ukrainian troops recaptured some towns and villages in the north-east.

While Russia’s advance in the vital industrial region of  Donbass has been slow, Western officials said that Russia has gained ground and taken some villages.

In a suburb of Kharkiv, Ukraine’s second-largest city, Russian rocket strikes killed one person and wounded three, the regional governor said.

Fighting across the east has driven thousands of Ukrainian civilians from their homes.