Danish Prime Minister Mette Frederiksen has announced early elections - hailing the next four years as "decisive" for her country.

"We as Danes and as Europeans will really have to stand on our own feet," Frederiksen told parliament. "We need to define our relationship with the United States," she added.

The past few months of Danish politics have been dominated by US President Donald Trump's threats to annex Greenland - a semi-autonomous Danish territory.

Last month, Trump dropped threats to use force against the fellow Nato ally to seize the Arctic island he argues is necessary to defend against possible attacks from Russia and China.

Announcing the decision to call early elections, Prime Minister Frederiksen told parliament that security "is and will remain the very foundation of Danish politics for many, many years to come".

"We must rearm to ensure peace on our continent," she said. "We must stick together in Europe, and we must secure the future of the Danish Commonwealth" - Denmark, Greenland and the Faroe Islands.

"Whether I will continue to be your prime minister depends on how strong a mandate you give the Social Democrats," Frederiksen said mentioning her party, the lead partner in the current Danish coalition.

Greenland is sparsely populated but its location between North America and the Arctic makes it well placed for early warning systems in the event of missile attacks.

Trump has claimed without evidence that it is "covered with Russian and Chinese ships all over the place".

More than 100 US military personnel are permanently stationed at a US base in Greenland's north-western tip - a facility that has been operated by the US since World War Two.

Under existing agreements with Denmark, the US has the power to bring as many troops as it wants to Greenland.

But Trump has not abandoned his plans - he has said previously that the US needs to own Greenland to properly defend it - and talks continue over ways of establishing US control over the entire island.

Both Denmark and Greenland have said they will not agree to cede sovereignty to the US. Their European allies have rallied to their support.

The latest confrontation may come in the next few days if Trump follows through with an announcement that the US plans to send a boat filled with medical supplies to Greenland, alleging "many people" on the island are sick and "not being taken care of".

Greenlandic Prime Minister Jens-Frederik Nielsen rejected the proposal, telling Trump that Greenland provides free healthcare for all citizens, unlike the US.