By Luke Baker

LONDON, March 12 (Reuters) - Four civilians died in an air strike in Afghanistan, Britain's defence ministry said on Wednesday, and a military expert said the tide of the war against insurgents was running against the United States and its allies.

Tuesday's strike was called in after Taliban militants ambushed British troops operating in the southern province of Helmand, the ministry said. Two women and two children were killed and a fifth person was injured.

"We deeply regret that this incident happened and do everything we can to mitigate this happening," a spokeswoman for Britain's Ministry of Defence said.

It was not immediately clear whether British aircraft or those of another force carried out the air strike.

A total of 1,977 civilians were killed in fighting in Afghanistan last year, including nearly 240 in air strikes by foreign troops, according to a leading Afghan NGO.

Afghan civilians have repeatedly accused British and American forces of being indiscriminate in their air strikes, with civilians frequently said to be among the victims.

Afghan President Hamid Karzai has voiced his anger and frustration at the number of civilian deaths, which have tended to fuel the local population's resentment of foreign troops.

"U.S. NEEDS HELP FROM ITS FRIENDS"

Lieutenant Colonel John Nagl, a U.S. military official and counter-insurgency expert, told a Stockholm security conference the tide of the Afghan conflict was running against the United States and its allies, in contrast to Iraq.

The turning of Sunni tribal leaders against al Qaeda, and the merging of their militia into government security forces, were important signs of progress, he said.

The trend in Iraq was "moving in our direction," said Nagl, whose 1st battalion of the 34th armoured regiment at Fort Riley, Kansas, trains U.S. transition teams that embed with Iraqi and Afghan security forces.

"And momentum matters in a counter-insurgency campaign because it's ultimately a struggle for the support of the people and the people can sense which way the tide is going," he said.

In Afghanistan, however, "the trends are not in the right direction. The number of suicide attacks was up dramatically in 2007, 2007 was a record year for opium production (which) obviously funds the larger Pashto-based insurgency."

Afghanistan has faced rising violence in the past two years, the bloodiest period since U.S.-led and Afghan forces overthrew the Taliban government in late 2001.

Washington is pressing reluctant European allies to do more to help combat a resurgent Taliban in the more volatile south and east of the country, an issue expected to loom large at NATO's April 2-4 summit in Bucharest.

More than 50,000 foreign troops are stationed in Afghanistan but the United States alone has more than three times that number in Iraq.

In New York, the top U.N. official for peacekeeping acknowledged the Afghan insurgency had been much worse than expected, and said a sharper U.N. mandate was needed if international efforts to stabilise Afghanistan were to succeed.

"We face an insurgency that has proven to be more resilient than we expected and more ruthless than we ever imagined," U.N. under-secretary-general for peacekeeping Jean-Marie Guehenno told the Security Council at an open debate on extending the U.N. mandate in Afghanistan.

Government institutions in Afghanistan remained fragile, he said, partly due to widespread corruption. The illicit opium trade continued to flourish, undermining the government by helping fund Taliban insurgents. (Additional reporting by Mark Trevelyan in Stockholm, Louis Charbonneau in New York) (Writing by Jon Boyle)

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