A US army helicopter, with four people believed to be on board, has crashed on a night mission in southwestern Afghanistan, a US defence official says.

In a brief official announcement following the crash on Thursday, the American military command in Kabul said there were "no confirmed reports" of casualties "at this time".

The announcement did not specify the nationality of the helicopter crew and said the cause of the crash was unknown.

Two US defence officials told the Associated Press news agency that four US troops were aboard the helicopter, identified as an Army Black Hawk, and one official said initial word from the scene was that officials "don't expect" that any of the four survived.

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Both officials spoke on condition of anonymity because details were still being collected and the families of the helicopter crew had not yet been notified.

Unspecified weather difficulties may have played a role in the crash, the two officials said, but it also was possible that enemy action was factor.

NATO's International Security Assistance Force (ISAF) said in a statement: "The crash site is secured; the cause is under investigation. Additional information will be released as appropriate."

In NATO terms, southwestern Afghanistan includes Nimroz and Helmand provinces, where nearly 20,000 US Marines are stationed.

A US defence official, speaking on condition of anonymity, told the Reuters news agency that the helicopter went down in the Regional Command-Southwest area, which includes the traditional Taliban stronghold of Helmand province.

Suicide attack

Earlier in the evening, a suicide blast in the same district of Helmand province led to the death of four members of the Afghan National Police (ANP), according to local media reports.

The attack occurred near the Lagrai bazaar area of Garamsair district when a suicide bomber detonated his explosives near a check point.

A local commander was among the four police officers killed in the attack. The explosion left another seven people wounded.

Qari Yousuf Ahmadi, a Taliban spokesman, said the group claimed responsibility for both the blast and helicopter crash.

While helicopter crashes occur with some regularity in Afghanistan, ISAF says they are rarely the result of Taliban fire.

Source: Agencies