The crash comes amid rising violence. At least 50 US troops have been killed in Afghanistan so far this month, putting July on track to be one of the deadliest months of the war for US forces.
In June, the Taliban shot down a helicopter in Helmand, killing four US troops.
Meanwhile in Kabul, the Afghan capital, Nato and Afghan forces captured a suspected insurgent who had allegedly planned attacks against this week's international conference in Kabul.
The man was detained on Wednesday night at a compound in Kabul after being suspected of planning to attack Tuesday's conference, the military coalition said in a statement.
The conference passed without major attacks, although rocket fire at Kabul airport forced the diversion of a plane carrying Ban Ki-moon, the UN secretary general, and Carl Bildt, the Swedish foreign minister.
Despite the relative success of the conference, the situation in Afghanistan seems to be getting worse.
More than 390 foreign soliders have been killed in the Afghan conflict so far this year, according to www.icasualties.org, an independent website.
Nato and the US have close to 150,000 troops in the country, with 30,000 deployed in the southern Taliban heartland, which includes Helamand province.