British forces are taking part in a major military offensive against the Taleban in southern Afghanistan.
Troops have secured and destroyed several Taleban compounds
Up to 3,000 Nato troops, including Afghans and US forces, are involved in the British-led Operation Silicon in the Sangin Valley, Helmand province.
BBC correspondent Alastair Leithead said the area was a Taleban stronghold and as troops moved forward, they were facing "considerable resistance".
The operation aims to drive Taleban fighters from the opium-producing area.
The BBC's correspondent said British troops had launched another offensive - Operation Silver - in Sangin town a few weeks ago and the current attack on the Taleban was an extension of that bid to push forward and gain ground.
More than 1,000 British troops, 1,000 soldiers from the Afghan National Army and personnel from the US, the Netherlands, Denmark, Estonia and Canada, are taking part in the operation.
Apache helicopters were involved and dozens of British armoured vehicles, which moved in at first light, had guns trained on targets in the "green area" - lush land used for growing poppies by the Helmand river, our correspondent said.
They were moving through the valley compound by compound, where the poppy harvest was under way, to try and secure the area and build defences, he added.
Lt Col Stephane Grenier, spokesman for the coalition, told Radio 4's Today programme that the mission was going well despite encountering "some Taleban resistance".
"The resistance has not been overwhelming - I would probably categorise it as mild to moderate," he said.
"So far, the troops who have been engaged in this latest manoeuvre have already secured and destroyed several Taleban compounds which is always good as we continue to erode Taliban extremists' ability to destabilise the south."
The operation is part of the wider Operation Achilles launched in March to prepare for reconstruction work in the area by the Afghan government.
A coalition statement said Nato-led forces and Afghan troops had also killed scores of Taleban fighters in the western province of Herat, near the border with Iran.
It said 87 had been killed in a 14-hour battle in Shindand district on Sunday - 49 died two days earlier.
About 30,000 coalition troops have been trying to reduce violence and boost the authority of President Hamid Karzai.
Nato and Afghan soldiers launched Operation Achilles, which was described as their largest offensive against the Taleban in the south, last month.
It involves more than 4,500 Nato troops, from Britain, the US, the Netherlands and Canada and nearly 1,000 Afghan soldiers in Helmand province.