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. The offensive is the Marines' first major operation since their recent deployment to Afghanistan. It is also the first such operation under President Barack Obama's presidency. The operation - codenamed Khanjar or Strike of the Sword - began when units moved into the Helmand river valley in the early hours of Thursday. Afghan map Helicopters and heavy transport vehicles carried out the advance, with Nato planes providing air cover. The BBC's Martin Patience in Kabul says the idea is that they will move into towns and villages which are under Taliban control. Because of a lack of capacity, troops have not been able to move into these areas. But with the fresh US deployments, military commanders say they are confident that they will make "significant" gains this summer, even if, as our correspondent says, a decisive victory is unlikely. UK-led forces in Helmand launched their own operation to combat the Taliban insurgency last week, in what the Ministry of Defence described as one of the largest air operations in modern times. Thousands of British forces under Nato command have been fighting the Taliban in Helmand since 2006, but there has been criticism that they have been overstretched and under-resourced. Security aim Southern Afghanistan is considered a Taliban stronghold. The security forces will build bases to provide security for the local people so that they can carry out every activity with this favourable background, and take their lives forward in peace Gulab Mangal Helmand Governor "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 Brig Gen Nicholson in a statement. At a briefing at the US military's Camp Leatherneck last week, he told personnel and embedded reporters: "One of the most critical things is to tell people why we're there, and we are going to have a limited opportunity to gain their trust."