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A Nato air strike has killed seven policemen in Afghanistan's northern Kunduz province, Afghan officials say.
The officers were mistakenly hit after a joint Nato-Afghan patrol was ambushed by Taliban insurgents, the officials told news agencies.
The Nato-led International Security Assistance Force (Isaf) said it was investigating the reports.
Isaf also said four Nato soldiers had died on Thursday in its big offensive, Operation Moshtarak, in the south.
Britain's Ministry of Defence confirmed that two Britons were among them.
Nine Nato soldiers have been killed since the operation in Helmand province began on Saturday. One Afghan soldier has died, while at least several dozen Taliban are believed to have been killed.
'Human shields'
About 15,000 Afghan and Nato troops, mostly US and British, are battling Taliban in and around the town of Marjah.
 |  I would be very cautious about any triumphalism just yet  Maj Gen Nick Carver Nato commander, southern Afghanistan  |
The area has long been known as a Taliban stronghold, and was considered a major hub of Afghanistan's opium poppy trade.
The operation is intended to oust the Taliban from the area and establish the control of the Kabul government.
British Maj Gen Nick Carter, in command of Nato forces in southern Afghanistan, said Operation Moshtarak, which means "together" in Dari, would take 25-30 days to secure the area.
More time would be needed after that to fully gauge the operation's success.
"Looking downstream, in three months' time or thereabouts we should have a pretty fair idea about whether we've been successful," he said.
"But I would be very cautious about any triumphalism just yet."
Nato and Afghan officials have claimed a degree of success in securing the area around Marjah but have run into stiff opposition from determined Taliban in the town itself.
 Nato has praised the conduct of Afghan troops in Operation Moshtarak |
Nato officials said they had learned from intercepted Taliban communications that the insurgents were running low on ammunition and have called for support.
Afghan commanders have said the Taliban in Marjah are using civilians as human shields to deter attacks from the US and Afghan troops.
On Sunday, 12 civilians died in a missile strike that Nato said had also killed three Taliban militants who were in the same house.
One villager who had fled to Helmand's provincial capital, Lashkar Gah, told BBC Pashto that relatives could not leave Marjah because the area was heavily mined.
Meanwhile, Isaf said it was investigating the reports from Kunduz province of the airstrike hitting Afghan police.
Afghan officials said the air strike was called in after a patrol of Afghan soldiers, police and Nato troops had been attacked by Taliban in Imam Sahib district.
The Afghan forces were hit by mistake, killing seven policemen and wounding two, a spokesman for the interior ministry said.