Moscow may respond to Western sanctions by opting out of the last nuclear arms deal with the United States, cutting diplomatic ties with Western nations, and freezing their assets, a senior Russian official warned.

The threat on Saturday by former President Dmitry Medvedev came as Russia’s ties with the West sank to new lows over its invasion of Ukraine.

Medvedev, deputy head of Russia’s Security Council chaired by President Vladimir Putin, also warned Moscow could restore the death penalty after Russia was removed from Europe’s top rights group – a chilling statement that shocked human rights activists in a country that has not had capital punishment for a quarter-century.

The sanctions placed new tight restrictions on Russian financial operations, imposed a draconian ban on technology exports to Russia and froze the assets of Putin and his foreign minister, a harsh response that dwarfed earlier Western restrictions.

Washington and its allies say even tougher sanctions are possible, including kicking Russia out of SWIFT, the dominant system for global financial transactions.

In sarcastic comments posted on a Russian social platform, Medvedev dismissed the sanctions as a show of Western "political impotence” that will only consolidate the Russian leadership and foment anti-Western feelings.

"We are being driven out of everywhere, punished and threatened, but we don’t feel scared,” he said, mocking the sanctions imposed by the US and its allies as an attempt to vindicate their past "shameful decisions, like a cowardly retreat from Afghanistan”.

Medvedev was placeholder president in 2008-2012 when Putin had to shift into the prime minister’s seat because of term limits. He then let Putin reclaim the presidency and served as his prime minister for eight years.

During his tenure as president, Medvedev was widely seen as more liberal compared with Putin, but on Saturday, he made a series of threats that even the most hawkish Kremlin figures have not mentioned to date.

Medvedev noted the sanctions offer the Kremlin a pretext to completely review its ties with the West, suggesting Russia could opt out of the New START nuclear arms control treaty that limits the US and Russian nuclear arsenals.

The treaty, which Medvedev signed in 2010 with then-US President Barack Obama, limits each country to no more than 1,550 deployed nuclear warheads and 700 deployed missiles and bombers, and envisages sweeping on-site inspections to verify compliance.

The pact – the last remaining US-Russian nuclear arms control agreement – had been set to expire in February 2021 but Moscow and Washington extended it for another five years.

If Russia opts out of the agreement now, it will remove any checks on US and Russian nuclear forces and raise new threats to global security.