Five US occupation troops Afghanistan were killed in a helicopter crash in bad weather in southern Afghanistan.

Monday night's crash brought the total number of U.S. troops killed that day to seven, making it the deadliest day for U.S. occupation forces so far this year. Two U.S. special operations forces were gunned down hours earlier in an insider attack by an Afghan policeman in eastern Afghanistan.

The NATO military coalition said in a statement that "initial reports" showed no ‘enemy’ activity in the area at the time. The cause of the crash is under investigation, the statement said.

A U.S. official said all five of the dead were American. The official said the helicopter went down outside Kandahar city, the capital of Kandahar province. The official spoke on condition of anonymity because the information had not been formally released.

The five dead included everyone aboard the UH-60 Black Hawk, said Maj. Adam Wojack, a spokesman for the international military coalition in Afghanistan.

Helicopter crashes are fairly frequent in Afghanistan, where the 100,000-strong international mission relies heavily on air transport.

Last August, seven American soldiers and four Afghans died when a Black Hawk helicopter crashed in Kandahar. Taliban militants claimed responsibility for bringing down the aircraft.

Monday's crash came on the same day that two US soldiers were killed and 10 wounded in a suspected insider attack in the eastern province of Wardak by a man in an Afghan army uniform who also killed several Afghan soldiers.