"Putin's announcement of sham referenda, partial military mobilisation and nuclear blackmail are a grave escalation," Borrell wrote on Twitter.

"Threatening with nuclear weapons is unacceptable and a real danger to all," he said.

The EU's top diplomat added: "the international community must unite preventing such actions. World peace is in jeopardy."

Borrell joined a chorus of condemnation from the West after Putin said Moscow would use "all the means at our disposal to protect Russia and our people" and ordered a partial military mobilisation, following a series of setbacks in Ukraine.

An EU spokesman had earlier said that Putin's announcements, along with a push for votes to annex occupied territories in Ukraine, showed his "desperation" after a series of military setbacks.

"This is just another proof of Putin that he is not interested in peace, that he's interested in escalating his war of aggression," said EU spokesman Peter Stano.

"This is also yet another sign of his desperation with how his aggression is going against Ukraine."

The EU would impose "consequences" on Moscow over the push to justify annexing areas of Ukraine it has occupied, Stano added.

But he refused to give any details, and EU member Hungary has expressed opposition to imposing more sanctions after six waves of unprecedented economic measures against Russia.

European diplomats told AFP that work was underway to come up with a new round of sanctions that could be put to the EU's 27 member states for approval.

The EU has said it will not recognise the "sham, illegal referendums" staged by the Kremlin in Ukraine. Stano said the bloc would continue its support to help Kyiv liberate its territory.

"Ukraine is fighting a justified war of self-defence for the survival of the nation," he said.

The EU has already disbursed 2.5 billion euros ($2.5 billion) for weapons deliveries to Ukraine and is preparing a fresh tranche expected to be worth an additional 500 million euros.