The United States and Germany on Wednesday announced deliveries of top-of-the-line tanks to Ukraine, sweeping aside their longstanding misgivings and signalling a new surge of Western support for an expected counter-offensive against Russian invasion.

In a televised address, President Joe Biden promised 31 Abrams tanks, one of the most powerful and sophisticated weapons in the US army. Shortly before, Chancellor Olaf Scholz gave the green light to Germany sending 14 Leopard 2 tanks -- a decision that opens the floodgates to several other European countries with Leopard stocks to send their own contributions.

Although Western countries have already sent Ukraine everything from artillery to Patriot anti-missile defense systems, tanks were long considered a step too far, risking a widening backlash from Russia.

But with Ukraine gearing up for a counter-offensive to push back increasingly entrenched Russians in the east and south of the country, the allies are now scrambling to send the vital weapon.

Flanked by his stern-faced secretary of state, Antony Blinken, and defense secretary, Lloyd Austin, Biden said that ramping up of Western armaments for Ukraine's military should not be seen as an attack on Russia.

"That's what this is about -- helping Ukraine defend and protect Ukrainian land. It is not an offensive threat to Russia. There is no offensive threat to Russia," Biden said.

Ukrainians, who have lobbied Western capitals for months to end restrictions on weapons enabling their troops to take on the more heavily armed Russians, were jubilant.

President Volodymyr Zelensky called the pledge of US Abrams "an important step on the path to victory."

Russia made it clear sending heavy tanks would cross a dangerous new line. Moscow's ambassador to the United States, Anatoly Antonov, said US approval for Abrams deliveries would be "another blatant provocation against the Russian Federation."

And Russia's ambassador to Germany, Sergei Nechaev, likewise warned that the "extremely dangerous decision takes the conflict to a new level of confrontation, and contradicts statements by German politicians about the unwillingness of the German Federation to get involved in it."

The news came as Russia was reported to be making advances on Bakhmut, an eastern Ukrainian town that has withstood months of assaults. Ukraine's military also told AFP that it had withdrawn from Soledar, another eastern town that Russia said it had captured earlier this month.

Need 'lot of Leopards'

Germany had been reluctant to send Leopards or even to allow other countries that had purchased the tanks to re-export them to Ukraine. US officials said they had not bowed to reported German pressure for Washington to pledge Abrams as a way to get Berlin to make a decision.

"Germany didn't force me to change my mind. I wanted to make sure we are all together," Biden told reporters, when asked about the coordinated announcement.

US officials had themselves been on the fence about sending Abrams, describing the tanks as overly complex for the Ukrainian military. Unlike the German tanks, which are ready to go, the M1 Abrams will have to be procured and will not arrive in Ukraine for months, a senior US official said.

The 31 tanks will add up to one Ukrainian battalion. And the twin decisions galvanized multiple offers of more, with Norway offering two Leopards on Wednesday and Spain saying it would look at what it could send from its own stocks of the German-made tank.

Poland has already promised to be a major provider.

In Kyiv, Zelensky's aide Yermak, said: "We need a lot of Leopards."

Biden spoke by phone about Ukraine with the leaders of Britain, France, Germany and Italy, the White House said.

NATO Secretary General Jens Stoltenberg "strongly" welcomed the decision by Germany.

And Polish Prime Minister Mateusz Morawiecki, who had vocally sought German approval to send on the tanks, thanked Berlin for the decision and called it "a big step towards stopping Russia."