By Caroline Wyatt
Defence correspondent, BBC News
British troops are fighting alongside Afghan forces
British forces in Afghanistan are taking part in a Nato ground and air offensive in Helmand province. It is the first major push since the US sent 30,000 extra troops to the country - but commanders are warning there will be casualties.
The Ministry of Defence has confirmed that a 'softening-up operation' is under way to clear the Taliban from its remaining strongholds in the area.
Codenamed Operation Moshtarak - which is Dari for "together" - the offensive involves British and American forces, as well as more than 2,500 Afghan personnel.
For several weeks, the troops have been working with local governors and tribal elders to prepare the way for a major assault on the Taliban's main bases.
Normally, military offensives rely on secrecy and the element of surprise.
However, this is a counter-insurgency operation. So, over the past few weeks, the coalition and Afghan security forces have been conducting what they call "shaping" operations.
Local people have been warned about what is to come so that they can protect themselves and stay away from the fighting.
By publicising the operation in advance, it is also hoped that some insurgents in Helmand may choose to lay down their weapons and change sides.
In the British-patrolled area of Nad Ali District, these have involved units from the Afghan National Army and the Afghan Police working alongside British soldiers from 1 Grenadier Guards Battle Group, 1 Royal Welsh Battle Group and others.
However, coalition commanders have warned that there will be fighting, and there are likely to be casualties.
British troops will focus on the areas between Nad Ali district centre and Lashkar Gah, the provincial capital, while American efforts will focus on the town of Marja, which is heavily linked to the opium trade.
The final aim of this offensive is to ensure that the areas that are cleared of Taliban fighters can also be held by coalition and Afghan forces and kept secure.
With a large number of US Marines also deployed in Helmand, British commanders say they now have the right density of troops to hold the ground after it is cleared of Taliban fighters.
They hope this will allow Afghan and international civilian efforts to rebuild governance to take root.