The United States army says it has launched a major offensive against the Taliban in the southern Afghan province of Helmand.
The US military says about 4,000 marines as well as 650 Afghan troops are involved, supported by Nato planes.
Brigadier General Larry Nicholson said the operation was different from previous ones because of the "massive size of the force" and its speed.
Officers on the ground said it was the largest marine offensive since the Vietnam War. It is the first such operation under President Barack Obama's presidency.
Southern Afghanistan is considered a Taliban stronghold.
A Taliban spokesperson said they would resist in various ways and that there would be no permanent US victory. Qari Yosuf Ahmadi said "a large number" of Taliban were in the area.
The operation began when units moved into the Helmand river valley in the early hours of Thursday, the BBC reports.
Helicopters and heavy transport vehicles carried out the advance, with Nato planes providing air cover.
"Where we go we will stay, and where we stay, we will hold, build and work toward transition of all security responsibilities to Afghan forces," said Brigadier General Nicholson in a statement.
The operation would have an initial highly aggressive stage lasting 36 hours, AFP news agency reported.
The offensive aims to improve security ahead of presidential elections on 20 August, allowing voter registration where before there was none, General Nicholson said.
As of June this year, Nato's International Security Assistance Force had 61,130 personnel from 42 countries including the US, Canada, European countries, New Zealand, Australia and Jordan. The US is the largest contributor, providing 28,850 soldiers.
