The United States and its allies have hit back at Russia's annexation of four Ukrainian regions, slapping sanctions on more than 1000 people and companies including arms supply networks as President Joe Biden warned Vladimir Putin he can't "get away with" seizing Ukrainian land.

The Russian annexation, though expected, escalated an already heated conflict that's become fraught with potential nuclear implications.

Biden said his administration would support any effort by Ukraine to retake the annexed territories by force, setting the stage for further hostilities. And Ukrainian President Volodymyr Zelenskyy announced that his country would make an "accelerated" bid to join the Nato military alliance, a plan not endorsed by the US or other allies that could add fuel to the fire.

"America and its allies are not going to be intimidated by Putin and his reckless words and threats," Biden told reporters. He added that Putin "can't seize his neighbour's territory and get away with it."

Russian President Vladimir Putin's announcement that Russia is incorporating four Ukrainian cities and areas was not unexpected following referendums this week that the West had denounced as shams. And the US and Western allies had previewed what their reaction would be.

But the developments dramatically increased tensions to a point not seen since the Cuban Missile Crisis 60 years ago during the Cold War.

Biden had spoken out against the annexation plans last week at the UN General Assembly, where a vast majority of other members also voiced support for respecting the territorial integrity of all nations. Today, he used the moment to reiterate that the US and Nato allies would not allow Russia to attack any of the nearby Nato members without facing a strong military response.

"America is fully prepared, prepared with our Nato allies to defend every single inch of Nato territory. Every single inch," Biden said. "And so, Mr Putin, don't misunderstand what I'm saying. Every inch."

While the Biden administration has identified the suppliers of Russia's weapons and battlefield high-tech as a priority, many of today's other sanctions were in line with penalties already enacted on thousands of Russian individuals and companies and may have comparatively little impact on the war effort. The administration hopes they will serve to undermine further support for Putin's invasion among Russia's elite.

Meanwhile, the US and its European allies are rushing to complete an agreement on a measure they hope will do more to damage Russia's economy: a cap on Russia's maritime oil exports that would undermine the prices Putin can demand his country's oil globally.

• Putin's troops face 'looming' defeat as Kremlin prepares to annex Ukrainian regions
• Aerial photos show incredible queues: More than 194,000 Russians flee to neighbouring countries

For now, Biden said the new US financial penalties, similar to those coming from like-minded countries, will impose severe costs on people and companies "that provide political or economic support to illegal attempts to change the status of Ukrainian territory." The sanctions will apply to countries, people or firms that support or do business with Russia-backed authorities in the newly annexed areas.

People watch on a large screen as Vladimir Putin delivers his speech after a ceremony to sign the treaties for four regions of Ukraine to join Russia in the Moscow's Kremlin. Photo / AP

Secretary of State Antony Blinken, echoing Biden, said the US "unequivocally rejects Russia's fraudulent attempt to change Ukraine's internationally recognised borders ... This is a clear violation of international law and the United Nations Charter."

"No one is fooled by what Moscow has done," Blinken told reporters at a joint news conference with Canada's visiting foreign minister. "The entire process around these sham referenda was a complete farce. This territory is and will remain Ukraine, and Ukraine has every right to defend its land, to defend its people and to take back the territory that Russia has seized from it."

This suggests the US will support the Ukrainians with weapons and ammunition to help them with military action to retake the annexed areas. The US has previously warned Ukraine not to use American weapons against Russian territory.

Blinken also spoke out against Putin's nuclear threats.

"This kind of loose talk about nuclear weapons is the height of irresponsibility, and it's something that we take very seriously," he said. "To date," he said, the US has not seen that "Russia is actually doing anything that suggests they are contemplating the use of nuclear weapons".

"I can just tell you that we plan against every possible scenario, including this one."

Biden also pushed back against Putin's comments in which he accused the West of sabotaging Russia-built natural gas pipelines under the Baltic Sea to Germany. Nordic nations said the undersea blasts that damaged the pipelines this week and have led to huge methane leaks involved several hundred pounds of explosives.

The President said the US and allies are still working to determine who was responsible for the blasts but excoriated Putin over his accusation.

"Let me say this, it was a deliberate act of sabotage," Biden said. "And now the Russians are pumping out disinformation and lies."

White House national security adviser Jake Sullivan said that "not many" countries have the ability to carry out such an attack on the pipelines. He stopped short of charging Russia responsible but made clear the US is suspicious that they may be complicit.

"Russia has done what it frequently does when it is responsible for something, which makes accusations that it was really someone else who did it," Sullivan said. "We've seen this repeatedly over time."

As for a broader guarantee of protection, Ukraine has sought Nato membership for years but has not yet been admitted because of concerns about its domestic governance. As a result of Russia's invasion, Finland and Sweden have applied for fast-track admission into the alliance.

The White House said Sullivan had spoken with Nato Secretary General Jens Stoltenberg to highlight the US and Nato's "firm commitment to Ukraine's sovereignty and territorial integrity."

But the Biden administration isn't embracing Zelenskyy's push for an accelerated path to Nato membership. Sullivan said the US was focused on supporting Ukraine through "practical on-the-ground support". He added that "the process in Brussels should be taken up at a different time."

Earlier, after Putin's announcement, the White House and the State, Treasury and Commerce departments had announced the new sanctions in a series of coordinated statements. The sanctions expand on what have been escalating penalties against Russia and its proxies since the invasion began on February 24.

Treasury designated hundreds of members of Russia's parliament, leaders of the country's financial and military infrastructure and suppliers for sanctions designations that include asset freezes and bans on Americans doing business with them. The Commerce Department added 57 companies to its list of export control violators, and the State Department added more than 900 people to its visa ban list, making them ineligible for travel to the United States.

Since the start of the invasion, the US and European nations have imposed significant financial penalties on Russia, its leadership and wealthy oligarchs tied to Putin. The allies have gone after the central bank reserves that underpin the Russian economy and have severed many Russian banks from a vital global financial network called Swift.

The war is having a devastating impact on the global economy and has contributed to massive disruptions to supplies of energy and food throughout the world. The Organisation for Economic Cooperation and Development this week said the global economy is set to lose $2.8 trillion in output in 2023 because of the conflict.

Putin appears undeterred. He warned that Russia would never give up the absorbed regions — Donetsk, Luhansk, Kherson and Zaporizhzhia — and would protect them as part of its sovereign territory.

Both houses of the Russian parliament will meet next week to approve the treaties for the regions to join Russia.

- Matthew Lee Fatima Hussein and Aamer Madhani, AP