The United States, which has long provided Iceland with its only military force, has decided to withdraw most of its service members and all of its fighter jets and helicopters from the country later this year, the U.S. ambassador said Thursday.

Iceland's government, which had recently offered to pay some of the cost of its defense to try to keep U.S. forces here, said it regretted the decision.

In an interview with The Associated Press, U.S. Ambassador Carol van Voorst said she and Nicholas Burns, the U.S. under secretary of state, had informed Prime Minister Haldor Asgrimsson and Foreign Minister Geir H. Haarde of the unilateral decision on Wednesday.

But Washington also said it would continue to honor its 1951 agreement with Iceland requiring the United States, under the auspices of NATO, to provide this country's defense. U.S. and Icelandic officials were to hold talks about how that will be done, Van Voorst said.

"Icelandic authorities regret this decision by the United States government," Haarde said in a statement Wednesday.

In it, he informed Icelanders that the four U.S. jet fighters and several U.S. helicopters used in search-and-rescue missions would leave by the end of September.

He said the next round of talks between the two governments must "be expedited as it is urgent that the future of the defense arrangement be decided."

Van Voorst said she understood Iceland's disappointment, even though it never has had its own military and is a peaceful country with no known enemies. Given the war on terrorism, she said, all countries face some risk.

But she said Iceland was encouraged by the U.S. promise to remain committed to its defense agreement. "As we have for half a century, we will stand with Iceland on any threats that arise," she said.

Neither side was publicly discussing options. But the United States could theoretically keep a small number of service members at its U.S. Naval Air Station in Keflavik on a rotating basis and have U.S. fighters stationed in Britain be on call to defend Iceland. NATO warplanes in Europe also could play a role.

The air station has about 1,200 U.S. service members. When their families are included, the total is about 3,000.

During the Cold War, the station in Keflavik, a bleak volcanic area outside the capital of Reykjavik, played a key role for U.S. security, allowing surveillance planes to hunt for Soviet aircraft and submarines.

The base, which the U.S. military built in 1942-43, also proved valuable during World War II, providing a refueling point for aircraft flying to Europe.

But the Pentagon has long said it wanted to drastically reduce the size of the base, since it believes Iceland is no longer strategically important. The U.S. military, hard-pressed by deployments in Iraq and Afghanistan, also has been re-evaluating the size and location of its bases around the world.

The U.S. military spends about $250 million a year to operate Keflavik, including maintenance of the air station and international airport, officials said. The base and the airport share two runways.

When Haarde returned from talks about the base with the State Department last month, he said Iceland needs the protection of at least four U.S. F-15 fighter jets at Keflavik, given the world's unknown security risks.

But Haarde also said his government had proposed paying to maintain the airport, a bill that now goes to Washington. He also suggested Iceland begin paying for search-and-rescue helicopters that are needed as a backup for the F-15s.

Without U.S. warplanes, Iceland would be left with a small Coast Guard, which has two unarmed helicopters and an airplane.