(Adds NATO quote, details)

By Ismail Sameem

KANDAHAR, Afghanistan, May 9 (Reuters) - An air strike by Western forces killed 21 civilians, including women and children, in Afghanistan, a provincial governor said on Wednesday, the latest in a string of civilian casualties that has riled Afghans.

The incident, which brings to nearly 90 the number of civilian deaths blamed by Afghan officials on Western troops in the past two weeks, comes as President Hamid Karzai faces rising pressure to halt the bloodshed and find a way to start peace talks with Taliban insurgents.

The air strike on Tuesday night hit houses in a village in the Sangin district of southern Helmand province, where Western forces have been hunting Taliban militants in recent weeks, Helmand governor Assadullah Wafa told Reuters.

"Last night, NATO forces carried out an operation in Sangin and as a result of its bombing, 21 civilians, including women and children and men, have been killed," he said.

NATO said its security force had not been active in that area on Tuesday but Wafa may have referring to a clash involving U.S.-led coalition troops hunting Taliban near Sangin, in which one coalition soldier was also killed the same day.

A U.S. military spokesman said he had heard reports about civilians killed in this clash but his information was that the only other people killed were militants.

Wafa said he had no report of casualties among the Taliban.

Helmand is a Taliban stronghold and the key drug-producing region of Afghanistan, the world's leading source of opium.

INCREASING PRESSURE

While most civilian casualties in Afghanistan in recent years have been linked to the hands of Taliban militants, a rising number of deaths in "friendly fire" from Western troops has added to pressure on the government.

The U.S. commander for eastern Afghanistan, Army Colonel John Nicholson, apologised on Tuesday for the killing of 19 civilians by U.S. troops just over a month ago.

An inquiry is underway into UN and Afghan reports that around 50 civilians were killed in a coalition operation in the west late last month, one of several incidents that has prompted NATO to seek ways to change how it operates to minimise civilian casualties.

"We know that our ability to operate here in support of the government of Afghanistan is dependent on the support of the people of Afghanistan," NATO spokesman Nicholas Lunt told reporters on Wednesday.

"We know very well that civilian deaths and injuries undermine this goodwill and support."

SPLINTERING

The government also faces criticism over rampant corruption and a lack of progress in rebuilding the country, despite billions of dollars in aid since the Taliban were ousted in 2001.

Parliament's upper house called on Tuesday for increased efforts to negotiate with the Taliban and an end to Western military operations to hunt them.

In addition, a new political grouping that includes some government figures has called for some of the president's powers to be taken away and given to a new role of prime minister.

The Taliban have stepped up attacks in recent weeks, as both sides in the conflict look for a decisive advantage this year.

Separately, four civilians were killed on Wednesday when a suspected suicide bomber exploded his charge prematurely in the southeastern province of Paktika, officials said.

(Additional reporting by Sayed Sallahuddin in Kabul and Elyas Wahdat in Khost)

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