The Defence Cooperation Agreement (DCA) is a major step for a country that in March ended two centuries of neutrality to join NATO.

Signed by Stockholm and Washington in December, the deal would give the US access to 17 military bases and training areas in Sweden, and allow the storage of weapons, military equipment and ammunition in the country.

Opponents say the agreement should state outright that the Scandinavian country will not allow nuclear weapons on its territory.

"We want to see legislation that bans nuclear weapons from being brought onto Swedish soil," Green Party MP Emma Berginger told parliament during Tuesday's debate.

"Unfortunately, the government has chosen to sign an agreement that doesn't close the door to nuclear weapons, and therefore the Green Party is going to vote no to this agreement," she said.

Greens leader Daniel Hellden had said Monday that the agreement made Sweden "a target for nuclear weapons" since "we're going to have 17 bases where the Americans can store (military) materiel".

Prime Minister Ulf Kristersson's centre-right minority government, propped up by the far-right Sweden Democrats, has said the deal respects Swedish sovereignty.

"It is very clear that Sweden is a sovereign nation, and there is no other country that can force Sweden to have nuclear weapons on Swedish soil," Defence Minister Pal Jonson insisted.

- 'Naive' -

For the bill to pass Tuesday, it has to win a 75 percent majority among MPs present for the vote in the Riksdag, and also the votes of more than half of the legislature's 349 MPs.

The Left and Green parties, which both voted against Sweden's NATO membership, together hold just 42 seats -- not enough to block the agreement's adoption on their own.

Two Left Party MPs called the government's handling of the agreement "not just naive, but downright foolish" in an op-ed article in the Aftonbladet newspaper on Sunday.

They said the DCA made Sweden less safe because "US defence and security policy is based on nuclear weapons".

"Nuclear weapons are a threat to mankind. That the government hasn't demanded any guarantees against such a brutal weapon of mass destruction is terrifying," they wrote.

The Swedish Peace and Arbitration Association, one of the biggest critics of the move, said two successive Swedish governments insisted during the NATO application process that Sweden would have the same stance on nuclear weapons as neighbouring Denmark and Norway.

"But unlike Norway's and Denmark's DCA agreements, there is no clause in Sweden's agreement against nuclear weapons being brought into or placed in Sweden," the association's head Kerstin Bergea wrote in another op-ed.

In addition, Finland, which joined NATO in April 2023, "has a national law prohibiting nuclear weapons on Finnish territory and Finland's DCA agreement refers to this law".

A similar Swedish clause would "strengthen the Nordic region and contribute to a joint de-escalation vis-a-vis Russian nuclear weapons", Bergea said.

- Nukes in wartime? -

Sweden's prime minister made headlines last month when he opened the door for the possibility of nuclear weapons in the country during wartime.

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

For the two Left Party MPs, in their op-ed, "That's an incredible statement and is totally the opposite of what the Swedish people think and what Sweden has long stood for."

Bergea questioned whether Sweden would be able to put a brake on the United States.

"An agreement based solely on confidence is not enough in important matters such as these," she said.

Jonson, the defence minister, has said Sweden needed to strengthen its international cooperation "to defend our freedom and democracy".

"With the DCA, Sweden can receive early, swift and effective military support from the United States in a deteriorating security situation," he said last month.

"The agreement acts as a deterrent and is stabilising. It reduces the risk of war breaking out and makes Sweden safer," Jonson said.