KABUL, Feb 12 (Reuters) - At least 18 civilians were killed last week in air strikes by international forces in Afghanistan’s Helmand province, an initial United Nations inquiry has concluded.

American military officials say their aircraft have conducted around 30 air strikes in Helmand in the past week. A spokesman said they were looking into the inquiry.

“We are investigating the allegations and working diligently to determine whether civilians were killed or injured as a result of U.S. air strikes,” said Brigadier General Charles Cleveland.

The NATO-led military mission has deployed hundreds of troops to Helmand in a bid to help Afghan security forces in their war against Taliban insurgents.

American aircraft and special forces have also provided combat support, with at least one U.S. soldier wounded in recent fighting.

On Thursday and Friday air strikes in Helmand’s Sangin district killed as many as 18 civilians, mostly women and children, according to a U.N. statement released on Sunday.

The U.N. said the strikes had been conducted by “international military forces,” but only U.S. aircraft have been involved in recent coalition strikes, according to military officials.

Family members of victims at the regional hospital in Helmand’s capital, Lashkar Gah, demanded explanations.

“How could women and children be Taliban?” Majnoon, a resident of Sangin, who said 11 people were killed in his brother’s house in a strike on Thursday, told Reuters.

Mullah Qasem, a local leader in Sangin, said the government wanted justice for the families of the victims.

The U.N. said a Taliban suicide bomber also killed at least seven civilians in an attack in Lashkar Gah on Saturday.

“(The U.N.) reiterates the need for all parties to the conflict to strictly adhere to their obligations under international humanitarian law to take all feasible measures to protect civilians from harm,” the statement said.

Civilian casualties from both American and Afghan air strikes increased dramatically last year, according to the U.N.’s most recent report on threats to civilians.

(Additional reporting by Mohammad Stanekzai in Lashkar Gah, Afghanistan; Editing by Hugh Lawson)