The United Nations mission in Afghanistan voiced concern over increasing numbers of civilian casualties as a result of airstrikes by U.S. or government forces, following reports that nine were killed in an eastern province last week.

Air strikes have spiked steeply this year, in a strategy aimed at forcing Taliban militants to accept peace talks, with the number of bombs dropped by the U.S. air force almost doubling in the first six months, to nearly 3,000.

The UNAMA mission said it had received "multiple, credible allegations" that a strike hit the house of a teacher in the eastern province of Kapisa on Saturday, killing nine members of the same family, including three women and four children. Six others were wounded, it said.

"UNAMA reminds all parties to the conflict to uphold their obligations to protect civilians from harm," it said in a statement.

"The Mission repeats its earlier call for government forces to uphold their commitment to regular review of targeting protocols and ensure mitigation measures and compensation for victims."

Mohammad Radmanish, a defence ministry spokesman confirmed civilian casualties during a joint operation by Afghan and U.S. forces that involved air support, but gave no details. He said an investigation was underway.

The U.S. military in Kabul said it was reviewing information regarding the Kapisa incident and reiterated that it did all it could to avoid civilian casualties.

"We are aware of the UNAMA announcement regarding Kapisa as well as the Afghan government's statements, and that they're conducting their independent process," it said in an emailed statement.

"It isn't uncommon for insurgents to use these accusations to drive a wedge between the military and the population. We will provide updates as they become available."

The reports underlined one of the problems facing Gen. Scott Miller, the new U.S. commander in Afghanistan who took up his post this month and must balance the need to pressure the Taliban with the need to avoid civilian casualties.

United Nations' data shows a jump of 52 percent in the number of civilians killed or wounded in air strikes in the first six months of the year. The U.N. said 149 civilians were killed and 204 wounded in air attacks in the year's first half, with women and children comprising more than half the 353 casualties.

Since the figures were reported in July, the U.N. said it had recorded increasing numbers of civilian casualties from air strikes.