The figures are a grim reminder of how the nearly seven-year war has failed to stabilize the country and suggest that ordinary civilians are bearing a heavy toll, particularly from stepped-up militant attacks.
John Holmes, the world body's humanitarian affairs chief, said the insecurity was making it increasingly difficult to deliver emergency aid to dirt-poor Afghans hit by the global food crisis.
"The humanitarian situation is clearly affected and made worse by the ongoing conflict in different parts of the country," Holmes told reporters in Kabul during a visit.
"Most of those casualties are caused by the insurgents, who seem to have no regard for civilian life, but there are also still significant numbers caused by the international military forces," he said.
Holmes said U.N. figures show that 698 civilians have died as a result of the fighting in the first half of this year. That compares to 430 in the first six months of 2007, a rise of 62 percent.
Anti-government militants caused 422 of the recorded civilian casualties - 60 percent - while government or foreign troops killed 255 people, according to the U.N. numbers. The cause of 21 other deaths was unclear.
Holmes said the proportion of civilian casualties caused by security forces had dropped from nearly half last year and clashes had become less dangerous to ordinary Afghans.
"It is clear that the international military forces are making every effort to minimize civilian casualties and recognize the damage this does and want to deal with that," he said.
"Nevertheless these problems are still there, and we need to deal with them and make sure that the safety of civilians comes first and international humanitarian law is respected by everybody."
NATO's reaction to the U.N. figures was cool.
"The U.N. Human Rights rapporteur made an accusation (in May) that we had killed 200, and I said then that those numbers were far, far higher than we would recognize, and that is still the case," said Mark Laity, a spokesman for the alliance in Kabul.
He provided no alternative figures.
Afghan leaders including President Hamid Karzai have accused NATO and the U.S.-led coalition of recklessly endangering civilians by using excessive force, including airstrikes, in residential areas.
Foreign commanders insist they take all reasonable precautions to avoid killing innocents and say militants routinely fire on them from houses and flee into villages.
Holmes said he came to Afghanistan because the humanitarian situation was "serious and is getting worse."
Drought in parts of northern and western Afghanistan has exacerbated food shortages caused by rising global prices for staples such as wheat and rice.
Holmes said the U.N. was providing food aid to 2.5 million people but would soon join the government in appealing to international donors for more funds to expand the program.
He said U.N. agencies and aid groups were finding it hard to reach vulnerable communities because of the risk that its staff would be attacked and said the world body would try to negotiate "days of tranquility or humanitarian corridors" with militants so that aid could get through safely.
U.N. food convoys have suffered 11 armed attacks this year, including one on Sunday in which several trucks were burned, and lost a total of 340 tons of food, he said.