There is no end in sight for a massive anti-Taliban offensive in southern Afghanistan, which must continue until local authorities gain control of the insurgent-dominated region, the U.S.-led coalition said Sunday.

More than 10,000 soldiers have fanned out across southern mountain ranges, desert plains and opium fields to crush the Taliban in Operation Mountain Thrust, the largest military operation in Afghanistan since the U.S.-led invasion that toppled the Taliban regime in late 2001.

CountryWatch: Afghanistan

"The purpose of doing that is so we can extend the authority of the government ... in areas where the government hasn't been in years," Col. Tom Collins, chief U.S. spokesman in Kabul, told reporters. "Because of that there is no end date per se to Mountain Thrust."

American, British and Canadian troops have inflicted large numbers of casualties in recent days in some of the most intense fighting of the offensive. Authorities said more than 70 Taliban were killed in 24 hours of violence in southern Helmand province. Insurgent roadside bombings and a suicide attack killed 10 Afghan troops and civilians, and a coalition soldier died in a militant ambush.

Afghanistan is witnessing its deadliest spate of violence since the U.S.-led invasion. More than 800 people, mainly Afghans, have been killed since May, according to an Associated Press tally of coalition and Afghan figures.

Afghan President Hamid Karzai has criticized the coalition for killing too many civilians.

He has ordered inquiries into three clashes this week to determine if there were civilian casualties and, if so, who was responsible.

Collins said there was no evidence that coalition forces killed or wounded civilians in any of the incidents.

A coalition soldier was fatally wounded fighting insurgents Sunday in southern Zabul province, the U.S. military said in a statement. His nationality and identity were not immediately released.

A roadside bomb killed six Afghan soldiers and wounded three more patrolling Sunday in western Herat province, the Afghan military said.

A suicide bomber struck in the east, killing four Afghan civilians and wounding 23, including a journalist, near a military checkpoint in the Paktya provincial capital of Gardez, the Interior Ministry said.

"I was knocked down by a strong, heavy explosion and saw other people lying on the street," said the journalist, Elias Wahdat of the Pajhwok Afghan News Agency. "Smoke covered the area and I was shaking."

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