KABUL - The number of civilians killed or wounded in the Afghan war increased by 20 percent during the first 10 months of this year, compared with the same period last year, according to a U.N. report issued this week.

The top U.N. envoy to Afghanistan, Staffan de Mistura, said as the world body released its latest quarterly report that insurgents are likely to stage high-profile attacks in the months ahead.

"Before it gets better, it may get worse," he said.

The report concluded that the number of civilian casualties attributable to insurgents increased by 25 percent during the 10-month period. It said insurgent groups were responsible for killing or injuring 4,738 civilians during that period, while 742 were killed or wounded by Afghan and international troops - a drop of 18 percent.

In a statement Thursday on its Web site, the Taliban called the civilian casualty figures in the report "a propaganda stunt aimed at concealing American brutalities."

U.S. airstrikes, long controversial in Afghanistan because of the high incidence of civilian casualties associated with them, were the leading cause of civilian deaths by NATO forces, the report said. At least 162 civilians were killed in airstrikes and 120 were wounded during the 10-month period.

On Thursday, NATO said it was investigating reports that one of its units had mistakenly killed two Afghans in northwestern Faryab province.

The grim statistics come as U.S. military officials are claiming some success in their effort to halt the Taliban's momentum as the war enters its 10th year.

De Mistura said insurgent groups are likely to try to undermine NATO's sense of traction by staging spectacular attacks in the near future.

"We should be ready, I'm afraid, for the next few months, for some tense security environment," he said.

The quarterly report said the period between July and October saw a 66 percent spike in security incidents compared with the same time frame last year. Assassinations reached an all-time high in August, it said, with most attacks targeting civilians and Afghan police. Suicide attacks occurred an average of three times a week, most of them directed at NATO troops, police and Afghan government officials.

Five civilians were wounded in a suicide bombing Thursday in Kunduz City, in northern Afghanistan, NATO said in a statement.

The number of NATO troops killed this year also reached a new high, according to a tally kept by the Web site icasualties.org. At least 705 international troops were killed here this year, far more than the 521 killed in 2009, the previous record.

The report also said the United Nations "welcomes the spirit" of President Hamid Karzai's attempt to oust private security companies from Afghanistan, which he says have operated here with impunity for years. But it also expressed concern that the firms' disbandment "before security could be assured by Afghan authorities" would lead to "a withdrawal of many development projects and activities."