Belarus seen preparing attack amid fears of nuclear escalation

"This is a watershed moment," said von der Leyen.

"Another taboo has fallen," said EU foreign policy chief Josep Borrell, who noted with some trepidation that Russia had put forces responsible for nuclear combat on alert.

Russian banks would also be unplugged from the SWIFT international-payments grid, Russian planes would be locked out of Europe, and media outlets Russia Today and Sputnik would be banned in the EU, they said.

Belarusian president Alexander Lukashenko had promised Ukrainian president Volodomyr Zelensky by phone Sunday that he would hold back on sending in his forces until the outcome of talks between Russian and Ukrainian officials that are expected near the River Pripyat, on the Belarusian border.

"We can only hope Lukashenko will stick to his word" and exercise restraint, Ukrainian foreign minister Dmytro Kuleba said in a press briefing from Kyiv on Facebook.

Hours earlier EU and US diplomats were warned that "[Belarusian] special operations forces are boarding planes" as part of a note sent by Alina Frolova, Ukraine's deputy defence minister, and seen by EUobserver. "An airborne landing is being prepared" for "Landing sites: Kyiv and Zhotimyr [in north-west Ukraine]," the note said.

EU diplomats in Minsk were trying to confirm if the Belarusian soldiers were really preparing to fly out, an EU source, who asked to remain anonymous, said.

But Belarusian president Alexander Lukashenko appeared to be laying the groundwork for intervention in Ukraine in a speech he gave in Minsk on Sunday.

"Is he [Zelensky] pushing me to embark on a special operation?" asked Lukashenko, who laced his speech with a litany of bogus accusations: that Ukrainians were beating up Belarusian people in Ukraine; and that Zelensky had trained "bandits from the self-exiled opposition" to attack Belarus.

The direct involvement by a third state in the war, if it went ahead, would mark the largest escalation since Russia invaded Ukraine on Thursday.

Putin, speaking in a TV statement Sunday, dialled up the tension further by threatening a nuclear response to what he described as Nato's "aggressive statements against our country."

"I order to move Russia's deterrence forces to a special regime of duty," said Putin, putting his nuclear forces in a state of combat readiness.

Reacting to Putin's words, Kuleba suggested that a tactical nuclear strike on Ukraine would be a global "catastrophe." Kuleba also said Putin's threat was designed to put pressure on Ukraine to surrender in the Pripyat talks.

"There's nothing bad in talking as such," Kuleba said. But he added that Ukraine "will not capitulate" and "will not give up a single inch" of its territory.

Nato has repeatedly vowed to stay out of the Ukraine war, but it's nonetheless sending extra troops to eastern allies to deter Russian aggression.

Nato and EU countries are also shipping arms to Ukraine via Poland, as heavy fighting continues in Kyiv and other cities.

Germany on Sunday announced it was sending anti-tank and anti-aircraft missiles to Ukraine, breaking a historical taboo on military intervention in a region associated with some of the Nazi era's worst crimes.

"We specifically need anti-tank, anti-air, anti-missile weapons. We need more air-to-air and ground-to-air missiles to counter the invaders," Ukraine's Kuleba said.

International legion

Alongside state-level intervention, international volunteer brigades to help the country also appeared to be forming, and Ukraine has created what Kuleba called an "international legion" for Europeans or other foreign fighters who wanted to take part in its "territorial defence."

Kyiv had already received "many hundreds" of volunteer requests, Kuleba said. "Every embassy and every consulate of Ukraine will help. Their access to Ukraine will be facilitated," he said.

Sunday's new EU measures will mark the third wave of sanctions on Russia in almost as many days. But there is still debate on how many Russian banks to strike out of SWIFT — and about how to keep Russian oil and gas imports flowing amid spiralling energy prices that hurt ordinary consumers in Europe.

"Some [EU] countries are trying to create loopholes so they can take some measures with their left hand, but keep on trading with Putin with their other hand," Kuleba said.

"Stop earning money soaked in our blood," the foreign minister said. Kuleba called for a "full oil and gas embargo" on Russia. "That oil and gas now contains Ukrainian blood," he said.

Lukashenko spoke on Sunday after holding a constitutional referendum that entrenched his grip on power and deepened Russia ties. His indirect support for Putin's war was already leading to extra EU sanctions on Belarus, von der Leyen said Sunday.

Were Belarusian paratroopers to join the fighting, then massive Russia-type EU sanctions on Belarus would also be discussed, diplomatic sources said.