COPENHAGEN, Aug 7 (AFP) - Three activists from the environmental pressure group Greenpeace are trying to break into a radar station in Greenland to protest US missile defence plans, a spokesman for the group said told Denmark's Ritzau news agency Tuesday.

The trio were dropped by the group's ship Arctic Sunrise on Monday and were believed late Tuesday to be near the Thule radar base -- a former Cold-War listening station that could be instrumental if the controversial US plans are to go ahead.

The activists aimed to breach the perimeter fence at the northwestern station and personally hand over a letter of protest to the base's commander after he earlier refused to meet with them, said a spokesman for Greenpeace Denmark, Dan Hindsgaul.

The three -- a Dutchman, a Frenchman and an American -- were dropped with enough supplies to last them a few days, the environmental group said. It said they were near the base late Tuesday and had still not been picked up by the base's security.

Meanwhile, police arrested three other activists from the group after they unfurled a large banner reading "Stop Star Wars" on the huge island's Mount Dundas early on Tuesday. They were later released by police after being questioned.

A fourth member of the group, Jacob Hartman, escaped in an inflatable dingy, returning to the Greenpeace ship.

Hartman explained that the aim of the stunt was to convince people working at the base that the United States' planned missile defense system would trigger a new global arms race.

"We've seen it in the past and we hope that it won't happen again," he said.

The US missile defence system has faced international criticism, especially from Russia, which says that the plans will involve a breach of the 1972 Anti-Ballistic Missile treaty signed by Moscow and Washington.

But the government of President George W. Bush contends that the missile shield is imperative in order to protect the United States and its allies from the threat of attack by so-called rogue states including Iraq and North Korea.

The Thule base was commissioned in 1951 and served as an early warning station against possible Russian attack during the Cold War. It has since been modernised to enable it to carry on its role as a surveillance post for the northern hemisphere.