US forces launching a major offensive in the Afghan province of Helmand say that they have encountered fierce Taliban resistance.
Brig Larry Nicholson told the AFP news agency on Friday that his soldiers were in a "hell of a fight" and were facing "difficult challenges".
Elsewhere in Helmand US commanders say they have met little resistance.
US Marines say Operation Khanjar, or Strike of the Sword, is intended to seize the lower Helmand River valley.
More than 700 UK troops have also launched a major offensive against insurgent strongholds in southern Afghanistan.
The province of Helmand is considered to be the heartland of the Taliban insurgency.
It is also a major poppy-producing area for the production of opium, which helps fund their activities.
Correspondents say that it is no surprise that much of the initial phase of the operation by some 4,000 Marines has met with little resistance, because that is a common tactic by insurgents.
The hard part will be winning support in a region where there are few foreigners, correspondents say.
Marine Capt Bill Pelletier told the AP news agency that the lack of resistance in the mission's first phase could change in coming days.
He said that that the primary objective of the offensive was not to kill the Taliban but to win the hearts and minds of the local population, who remain suspicious of outsiders.
"We are not worried about the Taliban, we are not focused on them. We are focused on the people," Capt Pelletier said. "It is important to engage with the key leaders, hear what they need most and what are their priorities."
The US forces were ferried in by relays of helicopters on Thursday to the southern Helmand districts of Garmsir and Nawa. They also helped Afghan forces take Khanishin, near the border with Pakistan, officers said.
"Today Marines are continuing to move towards those objectives that are still out there and they are going to work to stabilise security in these areas," Lt Kurt Stahl told AFP.
The Marines are at the forefront of President Barack Obama's new plan to fight the insurgency in Afghanistan, with an emphasis on protecting the population ahead of presidential elections on 20 August.
"When Marines go out into towns, they are always looking for opportunities to talk to village elders and explain why they are here," Lt Stahl said.
"The intention is to understand each other, elders can express their concerns and an open flow of communication is secured."
The BBC's defence correspondent says that US commanders hope the offensive will help turn the tide in the current stalemate against the Taliban.
The aim is to "clear, hold and build" in areas under Taliban influence ahead of the presidential elections.
About 4,000 US and 650 Afghan troops deployed to Helmand river valley
Initial operations focused on villages of Nawa and Garmsir near the provincial capital Lashkar Gah
British operation recently recaptured the village of Babaji from the Taliban to the north of the region