Two British Nato soldiers were also killed by a bomb while on patrol north of Lashkar Gah, the British defence ministry said.
Casualties among foreign forces in Afghanistan have grown steadily in recent months and McChrystal's review of strategy had been anxiously awaited.
The general, who commands both US and Nato forces in the country, said: "The situation in Afghanistan is serious, but success is achievable and demands a revised implementation strategy, commitment and resolve and increased unity of effort."
McChrystal is expected to request extra troops in a couple of weeks, the Associated Press reported unnamed Nato officials as saying.
The US general already has 103,000 troops under his command, including 63,000 from the US. He will have 68,000 by the end of the year.
A further increase could be politically difficult for Barack Obama, the US president.
Al Jazeera's James Bays, reporting from the Afghan capital, Kabul, said: "One of his plans is to look at the amount of troops he currently has in the country and work out what all of them are doing.
"I think that the general would like to see more of the troops who are here moved to frontline fighting positions."
Bays also said that much of the focus of McChrystal's assessment would be on the Afghan security forces.
Reports indicate McChrystal wants the Afghan army and police force – now totalling more than 160,000 – to be nearly doubled to more than 300,000, Bays said.
Daoud Sultanzoy, a member of the Afghan parliament, told Al Jazeera that while military strength was important, both the US forces and the Afghan government needed to tackle the hardships faced by civilians.
"The same people who know the Americans are not here to eliminate them, they also know the Americans and the Afghan government should deliver better - not in the military scene but on the non-military scene and the civilian life of the people of this country," he said.
McChrystal's assessment coincides with comments by Admiral Mike Mullen, the chairman of the US joint chiefs of staff, that the US military's biggest problem in Afghanistan is its lack of credibility.
Writing in the latest issue of the Joint Force Quarterly publication, Mullen said: "Our messages lack credibility because we haven't invested enough in building trust and relationships and we haven't always delivered on promises.
"We hurt ourselves more when our words don't align with our actions ... We must be vigilant about holding ourselves accountable to higher standards of conduct and closing any gaps, real or perceived, between what we say about ourselves and what we do to back it up.
"I also hope we learn to be more humble, to listen more. Because what we are after in the end - or should be after - are actions that speak for themselves, that speak for us. What we need more than anything is credibility."
McChrystal's report was supposed to have been submitted on August 12, a week before the August 20 Afghan election, but concerns that it could be misperceived and made into a political issue meant it was delayed.
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