Three Australian commandos and a US soldier have been killed in a helicopter crash in southern Afghanistan, Nato officials say.

北约官员称，南阿富汗地区发生一起直升机坠机事件，三名澳大利亚突击队员和一名美军士兵因此丧生。

The crash happened at 0339 local time in Kandahar province. There was no indication of enemy involvement.

Australia's armed forces chief, Air Chief Marshal Angus Houston, said the three men killed were from the Special Operations Task Group.

It is the country's deadliest single incident in the nearly nine-year war.

Two other Nato soldiers were killed on Monday in separate bombings in the south, the alliance announced. It did not give their nationalities.

'Tragic¹ day'

Seven Australians were also injured in Monday's crash. Two were said to be in a serious condition.

Air Chief Marshal Houston described the area where the helicopter came down as ''very rugged²（高低不平的，崎岖的） ".

He said: ''It's demanding terrain³（地形，地势） for the helicopter, particularly if the weather wasn't 100%.''

The latest incident takes Australia's military death toll⁴（死亡人数） in Afghanistan to 16.

It comes just two weeks after two Australian soldiers were killed by a roadside bomb.

Australian Prime Minister Kevin Rudd said it was "a very heavy price to pay", as he paid tribute to the three men who died early on Monday.

''This is a tragic day for Australia and the Australian Defence Force,'' Mr Rudd told parliament.

He said the commandos had been involved in a recent operation that dealt a major blow to Taliban insurgents⁵（叛乱分子） in northern Kandahar.

They had done previous tours of Afghanistan.

It comes as Nato is preparing for a major operation in Kandahar - the heart of the Taliban-led insurgency⁶ - this summer.

Australia has some 1,550 troops serving in Afghanistan.

At least 59 international soldiers, including more than 35 Americans, have been killed this month, making it one of the deadliest periods for international troops.

Meanwhile, the number of UK service personnel killed as a result of the Afghanistan conflict since 2001 has hit 300, after a wounded marine⁷ died in hospital.

Some 9,500 British military personnel are in Afghanistan as part of a Nato-led force.