A roadside bomb hit civilians riding a tractor to a wedding in southern Afghanistan, killing at least 20 people, officials said Thursday.

Helmand provincial police chief Assadullah Sherzad said women and children were among the 21 dead and five wounded in Garmser district, where roadside bombs are frequently used to attack foreign and Afghan forces.

The Afghan Ministry of Defense said that the roadside bomb Wednesday morning killed at least 20 people.

Thousands of U.S. Marines and British soldiers are pushing into Helmand, one of the centers of the Taliban insurgency, in attempt to extend government control and ensure stability ahead of the Aug. 20 presidential elections.

The insurgents, who pledge to disrupt the vote, have markedly ramped up attacks and dramatically increased their use of roadside bombs this year.

The U.N. says civilian deaths in the escalating war soared by 24 percent during the first half of 2009 compared with the same period last year and blamed most of the casualties on Taliban attacks launched with little regard for civilian lives.

The U.N. Assistance Mission to Afghanistan also pointed to stepped-up military operations by the United States and its allies, especially airstrikes.

But the report said the number of civilians killed by the Taliban and other "anti-government forces" during the first half of the year was double those attributed to the U.S.-led coalition and Afghan government forces. The U.N. termed that a "significant shift" from 2007, when the coalition was responsible for 41 percent of civilian deaths.

July was the bloodiest month for the U.S. and NATO in the nearly eight-year war. At least 42 U.S service members and 31 from other international military forces were killed, according to military reports.

The toll for August rose to 11 as the U.S. military reported that one of its troops had been killed by a roadside bomb in western Afghanistan on Wednesday. NATO said the death came after its forces battled insurgents spotted placing roadside bombs, but then were hit by one such device themselves.