Hundreds of British troops swept into the lush poppy fields of southern Afghanistan Monday, drawing hostile fire at the start of a NATO operation to expel the Taliban from a valley stronghold.

More than 3,000 NATO and Afghan troops are participating in Operation Silicon, the latest effort to bring Helmand province under the President Hamid Karzai's control.

• More coverage of the struggle for stability is available in FOXNews.com's Afghanistan Center.

A long column of armored vehicles brought several hundred British soldiers to the Sangin Valley, near the town of Gereshk and Afghanistan's strategic ring road that links the cities of Kandahar and Herat.

"It is all part of a longer-term plan to restore the whole of Helmand to government control," said Lt. Col. Stuart Carver, a British commander. "You have to do it a piece at a time."

The British soldiers came under attack from mortar rounds and machine-gun fire after they fanned out to patrol on foot.

An Associated Press reporter traveling with the troops heard officers ordering British artillery units to respond. Three Apache helicopters flew overhead but didn't immediately open fire. There were no reports of casualties.

The operation will not touch Helmand's poppy fields, which supply much of the world's opium and its more potent derivative, heroin. That could antagonize the 2 million farmers whose livelihoods depend on growing poppy, something the alliance wishes to avoid.

In western Afghanistan, U.S.-led coalition and Afghan forces battled with Taliban insurgents over three days, leaving at least 136 suspected militants dead, a coalition statement said Monday.

The clashes in Herat province were the deadliest reported in Afghanistan since January and provoked angry protests by hundreds of villagers Monday, chanting "Death to America!"

Acting on intelligence about Taliban activity in Herat's Zerkoh Valley, coalition and Afghan forces attacked the insurgents and called in an airstrike, destroying seven Taliban positions and killing 87 fighters during a 14-hour engagement on Sunday, the statement said.

Another 49 Taliban were killed two days earlier by a combination of gunfire and an airstrike, it said, adding that a U.S. soldier also was killed in the engagement.

The coalition statement said there were no reports of civilians wounded in the two battles. It was not immediately possible to confirm the casualty figures independently.

On Monday, hundreds gathered in front of the police station and government headquarters in Shindand district where Zerkoh Valley is located, said district police chief Gen. Gul Aqa.

Aqa confirmed that the attack had killed "a large number of people" but did not have figure for the number of dead. Contrary to coalition claims, Aqa said the Afghan police and army were not involved in the clashes.

"The Americans carried out an independent operation in the Zerkoh," he said, adding that protesters were demanding to know why Americans did not inform Afghan forces beforehand.

Recent weeks have seen an surge in violence in Afghanistan after a winter lull, with Taliban-led militants stepping up attacks, and coalition and NATO forces launching a series of offensives against around the country.

The clashes in Herat appear to be the deadliest in the once-stable west of the country since the ouster of the Taliban regime in late 2001. Most of the fighting has been concentrated in the volatile south and east.

The fighting is also the deadliest reported nationwide since January, when NATO said that about 150 suspected Taliban crossing from Pakistan were killed by an airstrike and ground fire in eastern Paktika province.

Visit FOXNews.com's Afghanistan Center for complete coverage.