Thirteen civilians were killed by joint U.S. and Afghan airstrikes in northern Afghanistan over the weekend, the United Nations said on Monday, casting a pall over the country’s hopeful embrace of the Persian new year. 

The incident near Kunduz city in the province by the same name took place overnight on Friday to Saturday, the U.N. Assistance Mission in Afghanistan said in a statement.

“The Mission expresses serious concern that initial fact-finding indicates that 10 of those killed were children, part of the same extended family whom were displaced by fighting elsewhere in the country,” the statement said. Three more civilians were wounded. 

An investigation into the incident was ongoing, said a spokeswoman for the U.S.-led NATO mission. 

Last month, a major U.N. report said 2018 was the deadliest year yet for civilians in the war, which is now in its 18th year. Casualties of children from U.S. and coalition airstrikes have been increasing since 2014. 

Across the country, Afghans have been marking the Persian new year, or Nowruz, with celebrations that began on Thursday and continued for several days. But hopes for a peaceful start were dashed by a surge in violence on all sides, coming as peace talks intensify between Washington and the Taliban in an effort to chart an end to America’s longest war. 

Continuing an upward trend of record-breaking Afghan military losses, more than 30 pro-government forces were killed over the weekend in fighting with Taliban insurgents in the heavily contested Sangin district of Helmand province, according to Afghan media reports. 

The devastating violence during Nowruz could also foreshadow what the following months could look like. Experts say accelerated fighting is expected in the lead-up to any peace deal, part of a strategy on both sides to increase leverage for when they eventually sit at the negotiating table. 

Violence on Friday also took the lives of two American troops and members of Afghanistan’s elite commando force in fighting with the Taliban in the north; in a separate incident, the Islamic State targeted revelers near a Shiite shrine and cemetery in Kabul. 

In Kunduz, site of the deadly strikes, relatives of the victims held a protest and tried to walk through Kunduz city with the corpses hoisted in the air. They were prevented from doing so by security forces, according to several residents. 

A spokesman for the provincial governor said officials were unable to corroborate the U.N. findings because of security constraints. “It is too risky to go there,” Esmatullah Muradi said by telephone. 

Sayed Salahuddin contributed to this report.