The Scandinavian country has become one of Washington's closest European allies in the past two decades, having fought alongside the US in Iraq.

"The United States has reached out to Denmark, proposing a bilateral defence cooperation," Prime Minister Mette Frederiksen told reporters.

"The exact nature of this collaboration has not yet been defined but it could include the presence of US troops, material and military equipment on Danish soil," she added.

The negotiations have been in the works for "a long time" and are not a direct result of the current crisis between Russia and Ukraine, Frederiksen said.

But that crisis demonstrates the need for more cooperation, she added.

"It is clear that the situation in Ukraine illustrates very, very clearly that we cannot take our freedom, our peace and our security for granted", the prime minister said.

Norway and the Baltic states already have similar agreements with Washington, Copenhagen said.

"NATO and the United States are guarantors of our security. That is why we join forces with the United States when Western values such as democracy and freedom are under threat," said Danish Defence Minister Morten Bodskov, who was also present at Thursday's press conference.

- No base, no nukes -

Researcher Peter Viggo Jakobsen of the Royal Danish Defence College said Copenhagen's move was intended to send a signal to Russia.

"The aim of this exercise is to show the Russians that we can quickly reinforce the troops already present in the Baltic states and Poland", he told news agency Ritzau.

While the details of the cooperation remain to be hammered out, Denmark said there were no plans for a new military base -- its autonomous territory Greenland is already home to the Thule air base -- nor nuclear weapons on its soil.

"If the Americans say they want authorisation to put nuclear weapons on Danish soil, the answer is no", insisted Bodskov.

The United States announced in early February it was sending 3,000 US troops to Eastern Europe to support NATO forces, including 1,700 to Poland, which also has a bilateral defence deal with the US.

Denmark's main opposition party, the Liberals, said they were in favour of the new defence cooperation deal, while two parties allied with the government were critical.

"The United States are our allies, but cooperation must remain within the NATO framework", a senior member of the Socialist People's Party, Karsten Honge, wrote on Twitter.

Denmark said earlier this week it was boosting its military preparedness in response to Russia's "unacceptable military pressure" on Ukraine, increasing the readiness of a mobile NATO-operational battalion of 700-800 troops.

It also said it would also be ready to send two F-16 fighter jets to its Baltic island of Bornholm "if judged necessary".