Sweden joined Nato in Washington on Thursday, two years after Russia's full-scale war in Ukraine led it to rethink its national security policy and conclude that support for the alliance was the Scandinavian nation's best guarantee of safety.

Swedish Prime Minister Ulf Kristersson handed over the final documentation to the US government on Thursday, the last step in a drawn-out process to secure the backing of all members to join the military alliance.

"Good things come to those who wait," US Secretary of State Antony Blinken said as he received Sweden's accession documents from Kristersson.

Blinken said “everything changed” after Russia’s attack on Ukraine, citing polls showing a massive shift in Swedish public opinion on joining Nato.

"Swedes realised something very profound: that if Putin was willing to try to erase one neighbour from the map, then he might well not stop there."

Sweden will be covered by the alliance's common defence guarantee under which an attack on one member is regarded as an attack on all.

"Today is a truly historic day. Sweden is now a member of Nato," Kristersson said. "We will defend freedom together with the countries closest to us – both in terms of geography, culture and values."

Hakan Yucel, 54, an IT worker in the Swedish capital said of the accession: "Before, we were outside and felt a little bit alone ... I think that the threat from Russia, it's going to be much less now."

The Nordic country would add cutting-edge submarines and a sizable fleet of domestically produced Gripen fighter jets to Nato forces and be a crucial link between the Atlantic and Baltic.

Russia has threatened to take unspecified "political and military-technical counter-measures" in response to Sweden's move.

"Joining Nato is really like buying insurance, at least as long as the United States is actually willing to be the insurance provider," said Barbara Kunz, a researcher at defence think tank SIPRI.

Nato membership marks a clear break with the past, when for more than 200 years, Sweden avoided military alliances and adopted a neutral stance in times of war. (Reuters)