After five days of Operation Moshtarak, officers said progress had also been slowed because of multitudes of hidden bombs in the farming district of Afghanistan's Helmand province that drug lords and the Taliban have controlled for years.
In Washington, President Barack Obama will convene his war cabinet to assess the first major test of his "surge" raising the number of foreign troops in Afghanistan to 150,000 by August in a bid to end the eight-year war.
On the ground, the top Afghan general in the battle on Wednesday accused the Taliban of hiding behind human shields.
"They have taken people hostage. Our troops have seen them putting women and children on the roofs of houses and firing from behind them," said General Moheedin Ghori, commander of the 4,400 Afghan troops taking part.
"We have strict orders not to fire at civilian areas."
The US commander, General Stanley McChrystal, has ordered that civilians be protected in a strategy seeking to harness military might and development in order to crush the Taliban and establish Western-backed government control.
US, Afghan and Nato generals spent months planning the assault on the drugs and Taliban nexus of Marjah, home to around 80,000 people in the southern province of Helmand.
