Swedish Prime Minister Magdalena Andersson has told her country's parliament that she sees "a historic change in our country's security policy line" as Sweden prepares to seek membership of Nato.

"Sweden needs formal security guarantees that come with membership in Nato," Ms Andersson said during a parliamentary debate, adding that the country was acting together with neighbouring Finland.

The debate is expected to be a formality as there is a clear majority of legislators in favour of joining Nato.

Sweden is expected to formally seek membership in the 30-member military alliance later on Monday.

The move in Sweden, which has been outside military alliances since the Napoleonic Wars, came after Finland on Sunday announced that it too would seek to join Nato in the wake of Russia's invasion of Ukraine on February 24.

"Sweden is best defended within Nato," Ms Andersson said.

"Unfortunately, we have no reason to believe that the trend (of Russia's actions) will reverse in the foreseeable future."

On Sunday, the Swedish Social Democrats broke with the party's longstanding position that Sweden must remain non-aligned, paving the way for a clear majority for Nato membership in parliament.

The debate on Monday enables the Social Democratic government to demonstrate that there is huge support for joining Nato.

Out of Sweden's eight parties, only two smaller, left-leaning parties oppose it.

In Helsinki, US Senate Republican leader Mitch McConnell said on Monday that there is "very significant" support in Congress and he expects swift ratification, adding that the two Nordic countries will "bring a great deal to the Nato alliance".

"The goal of the United States is to do it as rapidly as possible," Mr McConnell said.

He hoped a vote could be held before the August recess and added that he also hoped that the United States would be "the first to ratify".

"With regard to the size of the vote, I think it will be very significant. Not unanimous, but very significant," the long-time Nato supporter said.

On Sunday, he and Republican senators Susan Collins, John Barrasso and John Cornyn stopped in Stockholm and met with Ms Andersson among others.

They made a surprise stop on Saturday in Ukraine's capital to express solidarity in the fight against the Kremlin.

Public opinion in both Sweden and Finland had been firmly against joining Nato, but support for membership surged almost overnight after the invasion of Ukraine started.