Swedish Prime Minister Ulf Kristersson said Monday he was open to allowing nuclear weapons on Swedish soil in wartime, as critics call for the new NATO member to ban their deployment.

Sweden's parliament is set to vote on a Defence Cooperation Agreement (DCA) with the United States in June which will give the US access to military bases in Sweden and allow the storage of military equipment and weapons in the Scandinavian country.

Sweden abandoned two centuries of military non-alignment to join NATO in March this year.

Calls have mounted in recent weeks, from the Swedish Peace and Arbitration Association among others, for the government to put in writing in the DCA agreement that Sweden will not allow nuclear weapons on its soil.

The government has repeatedly insisted there is no need to have a ban spelled out, citing "broad consensus on nuclear weapons" in Sweden as well as a parliamentary decision that bars nuclear weapons in Sweden in peacetime.

But Kristersson said on Monday that wartime was a different story.

"In a war situation it's a completely different matter, (it) would depend entirely on what would happen," he told public broadcaster Swedish Radio.

"In the absolute worst-case scenario, the democratic countries in our part of the world must ultimately be able to defend themselves against countries that could threaten us with nuclear weapons."

He insisted that any such decision to place nuclear weapons in Sweden would be taken by Sweden, not the United States.

"Sweden decides over Swedish territory," he said.

But, he stressed, "the whole purpose of our NATO membership and our defence is to ensure that that situation does not arise."

If Ukraine had been a NATO member, "it would not have been attacked by Russia," he said.

Sweden's Social Democratic Party, which was in power when Sweden submitted its NATO membership application in May 2022, said at the time it would work to express "unilateral reservations against the deployment of nuclear weapons and permanent bases on Swedish territory".

Nordic neighbours Denmark and Norway, which are already NATO members, have both refused to allow foreign countries to establish permanent military bases or nuclear weapons on their soil in peacetime.