US Senators have moved toward what is expected to be an overwhelming, bipartisan vote today approving Nato membership for Finland and Sweden, calling expansion of the Western defensive bloc a "slam-dunk" for US national security and a day of reckoning for Russian President Vladimir Putin over his invasion of Ukraine.

Senators invited the ambassadors of the two Nordic nations to witness the debate and the vote, a crucial step in opening a new era for the now 30-member North Atlantic Treaty Organisation and its 73-year-old pact of mutual defence among the United States and democratic allies in Europe.

US President Joe Biden has sought quick entry for the two previously non-militarily aligned northern European nations. Their candidacies have won ratification from more than half of the Nato member nations in the roughly three months since the two applied, a purposely rapid pace meant to send a message to Russia over its six-month-old war against Ukraine's West-looking government.

"It sends a warning shot to tyrants around the world who believe free democracies are just up for grabs," Senator Amy Klobuchar said in the Senate debate ahead of the vote. "Russia's unprovoked invasion has changed the way we think about world security."

Senate Minority Leader Mitch McConnell, who visited Kyiv earlier this year, urged unanimous approval, and the vote was expected to near that. Speaking to the Senate, McConnell cited Finland's and Sweden's well-funded, modernising militaries and their experience working with US forces and weapons systems, calling it a "slam-dunk for national security" of the United States.

"Their accession will make Nato stronger and America more secure. If any senator is looking for a defensible excuse to vote no, I wish them good luck," McConnell said.

Republican Senators Tom Cotton and Josh Hawley on Capitol Hill in Washington DC. Photo / AP

Senator Josh Hawley, a Missouri Republican who often aligns his positions with those of the most ardent supporters of former President Donald Trump, has been one of the few to speak in opposition. Hawley took the Senate floor to call European security alliances a distraction from what he called the United States' chief rival — China, not Russia.

"We can do more in Europe ... devote more resources, more firepower ... or do what we need to do to deter Asia and China. We cannot do both," Hawley said, calling his a "classic nationalist approach" to foreign policy.

Senator Tom Cotton of Arkansas, like Hawley a possible 2024 presidential contender, rebutted Hawley's points without naming his potential Republican rival.

That included arguing against Hawley's contention that a bigger Nato would mean more obligations for the US military, the world's largest. Cotton was one of many citing the two nations' military strengths — including Finland's experience securing its hundreds of miles of border with Russia and its well-trained ground forces, and Sweden's well-equipped navy and air force.

They're "two of the strongest members of the alliance the minute they join," Cotton said.

US State and Defense department officials consider the two countries net "security providers," strengthening Nato's defence posture in the Baltics in particular. Finland is expected to exceed Nato's 2 per cent GDP defence spending target in 2022, and Sweden has committed to meet the 2 per cent goal.

That's in contrast to many of Nato's newcomers formerly from the orbit of the Soviet Union, many with smaller militaries and economies. North Macedonia, Nato's most recent newcomer nation, brought an active military of just 8,000 personnel when it joined in 2020.

Senate Majority Leader Chuck Schumer said he and McConnell had committed to the country's leaders that the Senate would approve the ratification resolution "as fast as we could" to bolster the alliance "in light of recent Russian aggression".

US Senate Majority Leader Chuck Schumer welcomes Finnish and Swedish diplomats to Capitol Hill in Washington DC. Photo / AP

A late-afternoon vote was expected after debate on the measure and amendments. One amendment from Republican Senator Rand Paul would ensure that Nato's guarantee to defend its members does not replace a formal role for Congress in authorising the use of military force. Another from Republican Senator Dan Sullivan would declare that all Nato members should spend a minimum of 2 per cent of their gross domestic product on defence and 20 per cent of their defence budgets on major equipment, including research and development.

Sweden and Finland applied in May, setting aside their longstanding stance of military nonalignment. It was a major shift of security arrangements for the two countries after neighbouring Russia launched its war on Ukraine in late February. Biden encouraged their joining and welcomed the two countries' government heads to the White House in May, standing side by side with them in a signal of US backing.

The US and its European allies have rallied with newfound partnership in the face of Russian President Vladimir Putin's aggression, strengthening the Nato alliance first formed after World War II.

"Enlarging Nato is exactly the opposite of what Putin envisioned when he ordered his tanks to invade Ukraine," Senator Bob Menendez, a New Jersey Democrat and chairman of the Senate Foreign Relations Committee, said Wednesday, adding that the West could not allow Russia to "launch invasions of countries".

Biden sent the protocols to the Senate for review in July, launching a notably speedy process in the typically divided and slower-moving chamber.

Each member government in Nato must give its approval for any new member to join. The process ran into unexpected trouble when Turkey raised concerns over adding Sweden and Finland, accusing the two of being soft on banned Turkish Kurdish exile groups. Turkey's objections still threaten the two countries' membership.

- Ellen Knickmeyer and Lisa Mascaro, AP