In rallying global arms supplies and asking Congress for $33 billion more to support Ukraine, Washington is choosing to ignore Vladimir Putin's threats to use nuclear arms, and instead is openly testing the Russian leader's limits.

After the United States brought 40 countries to a US base in Germany last week to discuss aid for Ukraine's war with Russia, Putin on Wednesday threatened a "lightning fast" response if there is any direct intervention by outsiders on Kyiv's behalf.

"We have all the tools for this, that no one else can boast of having. We won't boast about it: we'll use them, if needed," Putin said.

It was a thinly veiled reference to Russia's tactical nuclear weapons, which Russian military doctrine holds can be used to force an adversary to retreat.

Rather than pull back at that threat to unleash Moscow's nuclear bombs, US President Joe Biden doubled down on US support for Ukraine.

The $33 billion he requested includes $20 billion for arms and munitions, nearly seven times the amount sent to the country since the Russians invaded on February 24.

- No longer hiding support -

Early in the war Washington worried that aggressively equipping Ukraine's army with anything but "defensive" weaponry risked embroiling the US and NATO in a direct conflict with Russia.

Now, the Pentagon has shed those earlier inhibitions and is shipping offensive weapons like heavy artillery, helicopters and attack drones.

Rather than hiding it, the Pentagon also began openly talking this week about how it is training Ukrainian troops, including inside Germany, to use the weapons they are receiving.

And instead of saying, as it did in February, that it wants only to help Ukraine survive, Washington now says its goal in the war is to debilitate Russia for the long term.

"We want to see Russia weakened to the degree that it can't do the kinds of things that it has done in invading Ukraine," US Secretary of Defense Lloyd Austin said after a visit to Kyiv last week.

- Shrugging off Moscow's threat -

Inside the US government, Putin's nuclear threat is being shrugged off.

Biden and his top officials have labeled Moscow's nuclear threats "irresponsible."

"It shows the desperation that Russia is feeling about their abject failure in being able to do what they set out to do" in Ukraine, Biden said Wednesday.

On Friday, a senior US defense official said that even as the Pentagon constantly monitor's Russia's nuclear force, "We do not assess that there is a threat of the use of nuclear weapons and no threat to NATO territory."

Lawrence Freedman, professor emeritus at King's College in London, said there are no signs that Russia is preparing to use its nuclear or chemical weapons in the Ukraine conflict.

"Its threats are taken less seriously than before and it has nothing positive to offer. It is already a diminished power," Freedman wrote on his blog.

That view was echoed by Gideon Rose of the Council on Foreign Relations in New York. 

"Moscow will not use nuclear weapons during the conflict," said Rose.

Putin "knows that extraordinary retaliation and universal opprobrium would follow, with no remotely comparable strategic upsides to justify them," he said.

Even if Biden has repeatedly said that US troops will not get directly involved in Ukraine, the surge in support has turned the conflict into a proxy war, Sam Winter-Levy, a warfare specialist at Princeton University, wrote on the website "War on the Rocks."

"The West needs to be clear-eyed about the situation it confronts. It is currently waging a proxy war with Russia -- one that poses very real risks of escalation," he wrote.

Nevertheless, he said the current path "may still be the best option."

"Ultimately, the only options worse than a proxy war are a cheap Russian victory in Ukraine — or a direct confrontation between Russia and the United States," he said.

Latest developments

Here are the latest developments in the war in Ukraine:

- Missile hits Odessa airport -

A Russian missile destroys the runway at Odessa airport in southern Ukraine but there are no victims, the regional governor Maxim Marchenko says on his Telegram account.

- Civilian group leaving Mariupol -

A group of 20 civilians are leaving the Azovstal steelworks, where the last Ukrainian troops are holed up in the Black Sea port of Mariupol, the soldiers there say.

- Russian plane enters Swedish airspace: Stockholm -

A Russian reconnaissance plane briefly violated Sweden's airspace, say Swedish defence officials, as the Scandinavian country ponders a bid for NATO membership after Russia's invasion of Ukraine.

- Ukrainians freed in prisoner swap -

Fourteen Ukrainians including a pregnant soldier have been freed in the latest prisoner exchange with Russian forces, Ukraine says, without revealing the number of Russians returned to Moscow.

The exchange of seven military personnel and seven civilians included a soldier who is five months pregnant, Deputy Prime Minister Iryna Vereshchuk announces on Telegram.

- More bodies found near Bucha -

The bodies of three men shot in the head and with their hands tied have been found in a pit near Bucha, a town close to Kyiv, police say.

There were also traces of torture, says the statement. Bucha has become synonymous with allegations of Russian war crimes since dozens of bodies were first discovered there in early April.

- France to 'intensify' aid -

French President Emmanuel Macron says France will "intensify" its supply of military and humanitarian support following a conversation with his Ukrainian counterpart Volodymyr Zelensky.

- Kharkiv shelled again -

Ukraine's second city Kharkiv is hit by multiple Russian shellings, though President Zelensky says Ukrainian forces are making "tactical successes" in the region.

One person was killed and five injured, Kharkiv's regional military administration says on Telegram.

- Germany to 'reconsider' Schroeder's perks -

The German government will consider withdrawing perks given to ex-chancellor Gerhard Schroeder because of his close ties to Russian President Vladimir Putin, Germany's finance minister says.

- 13 million uprooted -

More than 5.4 million Ukrainians have fled their country since Russia invaded two months ago, with tens of thousands joining their ranks every day, the United Nations says.

Beyond the refugees, the International Organization for Migration (IOM) estimates more than 7.7 million people have been displaced within Ukraine, meaning that more than 13 million people overall have been uprooted by the conflict.

- Angela Jolie visits Lviv -

Hollywood star Angelina Jolie makes a surprise appearance in the western Ukrainian city of Lviv where she meets displaced people.

Jolie is a UNHCR special envoy, but it was not known if she was visiting the country in that capacity.