Officials earlier told Al Jazeera that "possibly more than two" passengers were killed in the crash.

In depth

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Fazlul Haq, the district chief of Sangin, said he saw the helicopter before it crashed.

"We are aware of an incident involving a non-British military helicopter in Helmand province," Lieutenant-Colonel Nick Richardson of the Nato-led force told the Reuters news agency by phone from Helmand.

A spokeswoman for the US navy, Lieutenant-Commander Christine Sidenstricker, also acknowledged there had been some casualties.

Helmand is the site of an offensive by around 4,000 US soldiers backed by hundreds of Nato and Afghan forces aimed at driving out the Taliban.

It is part of the main bastion for Taliban fighters and is the main drug-producing region of Afghanistan.

The US-led operation is the biggest by foreign troops since they removed the Taliban from power in 2001.

Crucial elections

The offensive comes in advance of next month's presidential election, which is crucial both for Kabul and for a US administration that has identified Afghanistan and neighbouring Pakistan as its top foreign policy priority.

Zeina Khodr, Al Jazeera's correspondent in Kabul, said: "Security is proving to be a major challenge in the upcoming presidential elections.

"According to the interior ministry, 134 out of 364 districts in the country are deemed unsafe."

"There are more than 28,000 polling stations across Afghanistan, and according to the free and fair election foundation here, 30 per cent of those polling stations will not be secured."

Bloodiest month

Taliban fighters have stepped up their attacks in different parts of Afghanistan against foreign soldiers and the government, making July the bloodiest month for foreign troops for nearly a year.

Two US soldiers from the Isaf were killed on Monday in Helmand, a spokesman for the US military said.

Prior to that, at least 15 foreign soldiers had been killed since the start of the assault in Helmand.

Last week, three foreign soldiers were killed when a Nato helicopter crashed in the southern Zabul province.

Several dozen Taliban fighters have also been killed in Helmand, according to the Afghan government, but there has been no major engagement with the fighters there.