A civilian helicopter carrying supplies to Nato-led forces has crashed in southern Afghanistan, killing six Ukrainians on board and a child on the ground, officials have said.

The helicopter erupted into flames on Tuesday in the Sangin district of Helmand province, where thousands of US, Afghan and British troops are conducting an offensive against the Taliban.

The Taliban said its fighters had shot down the helicopter with "anti-aircraft fire".

Authorities in the former Soviet republic of Moldova also said that the helicopter, owned by an aviation company there, was brought down by a missile or rocket.

But Nato officials in Kabul, the Afghan capital, refused to confirm the cause of the crash saying it was still under investigation.

Civilian deaths

"At around 9am (04:30 GMT) this morning, a private helicopter crashed outside Sangin military base ... we have at least six people killed," a spokesman for the Nato-led International Security Assistance Force (Isaf) said on Tuesday.

Daud Ahmadi, the spokesman for the governor of Helmand, said a six-year-old child on the ground was also killed, The Associated Press news agency reported.

About 4,000 US marines, backed by hundreds of Nato and Afghan troops, are fighting to seize areas of Helmand from the Taliban.

The province is a main bastion for Taliban fighters and the world's largest drug-producing region.

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

The offensive comes in advance of next month's presidential election in Afghanistan.

The election is crucial both for Kabul and for a US administration that has identified Afghanistan and neighbouring Pakistan as its top foreign policy priority.

'Major challenge'

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."

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.

At least 15 foreign soldiers have been killed since the start of the assault in Helmand, and two US soldiers were killed on Monday in Helmand.

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.