Legendary former US Senator Ted Stevens was killed when a small plane carrying nine people slammed into a remote Alaska mountainside leaving five passengers dead, officials said Tuesday.

Stevens, 86, the longest-serving Republican in Senate history before being voted out of office in 2008, was traveling on a flight which also included EADS North America chief Sean O'Keefe, the former boss of US space agency NASA.

O'Keefe, 54, and his teenage son were among four who survived the crash, which occurred on Monday night in poor weather near Aleknagik, about 300 miles (480 kilometers) southwest of Anchorage.

The cause of the crash was not known Tuesday as officials from the National Transportation Safety Board began an investigation at the crash site.

President Barack Obama led the tributes to Stevens, a decorated World War Two pilot who had already cheated death in a 1978 plane crash in Anchorage which claimed the life of his first wife, Ann.

"Senator Ted Stevens devoted his career to serving the people of Alaska and fighting for our men and women in uniform," Obama said in a statement. "Michelle and I extend our condolences to the entire Stevens family and to the families of those who perished alongside Senator Stevens in this terrible accident."

Vice President Joe Biden, who served with Stevens in the Senate for more than 30 years, echoed the president's condolences.

"As senators, he and I bonded over shared similar family tragedies, but we also celebrated life's great moments together too, like the births of our daughters a few days apart," Biden said. "He was my friend and I will miss him."

Stevens was flying to the Agulowak Lodge, owned by the GCI telecommunications company which also owns the single engine de Havilland Canada DHC-3 Otter seaplane that crashed.

Stevens was lost his re-election bid in November 2008 elections, just one week after he was found guilty of corruption regarding gifts he received from an oil services firm. The verdict was later overturned.

Former Alaska governor Sarah Palin described Stevens as a "warrior and a true champion of Alaska."

"In our land of towering mountains and larger than life characters, none were larger," Palin said in a statement. "Our hearts and prayers are with the Stevens family and the families of the other victims of the crash."

European aerospace giant EADS said in a statement O'Keefe and his teenage son were among the survivors.

A friend of O'Keefe's, who headed NASA during a turbulent four year period between 2001 and 2005 which included the 2003 Space Shuttle Columbia disaster, said he had suffered a broken pelvis and "a lot of scrapes and burns."

"They did both survive... a lot of injuries and so forth but they are alive," the family source told AFP. O'Keefe's son suffered a broken leg, cuts and burns, the source added.

The Alaska National Guard said personnel reached the scene of the crash Tuesday after wreckage was spotted on Monday night.

John Bouker, the pilot who first spotted the crash site on Monday, said the plane appeared to have hit the mountain.

"He bounced up the mountain," Bouker told the Anchorage Daily News. "He looked like he was in a full power climb... He looked like he was climbing when he hit."

The wilderness area in which the plane went down is known for its startling beauty. Deep forests of fir trees cover thousands of square miles of landscape, where runoff from sweeping mountains serves a series of large lakes, including Lake Aleknagik, near the Agulowak Lodge where Stevens was headed.

Transport by small plane including seaplanes is common in Alaska, where weather, long distances and an incomplete highway system make road travel difficult.

The crash is the third in less than two weeks in the sparsely populated state. Three people died earlier this month when a twin-engine cargo plane crashed in Denali National Park, and a US military C-17 Globemaster cargo plane crashed on a training mission late last month at the Elmendorf base near Alaska's biggest city Anchorage, killing all four crew members aboard.