The number of US drone strikes in Afghanistan rose sharply last year compared with 2011, the United Nations said Tuesday.

The increase was a sign that drones are taking a greater role as Americans try to streamline the fight against insurgents while preparing to withdraw combat forces in less than two years.

Drones have become a major source of contention between the US and countries like Pakistan, where covert strikes on militant leaders have drawn condemnation and allegations of sovereignty infringements as family members and other bystanders are killed.

They have not been a prominent issue in Afghanistan, however. While drone attacks have occurred, they have largely been in support of ground troops during operations and have not been singled out by President Hamid Karzai's administration in its campaign against international air strikes.

The steep rise in the number of weapons fired from unmanned aerial aircraft – the formal term for drones – raises the possibility that may change as US forces become more dependent on such attacks to fight al-Qaida and other insurgents as combat missions are due to end by the end of 2014.

The UN mission in Afghanistan (Unama) said 506 weapons were released by drones in 2012, compared with 294 the previous year. Five incidents resulted in casualties with 16 civilians killed and three wounded, up from just one incident in 2011.

Georgette Gagnon, the head of human rights for Unama, said it was the first year the UN had tried to document civilian casualties from drones.

The US air force central command also recorded an increase, giving the numbers of weapons released by drones as 243 in 2009, 277 in 2010, 294 in 2011 and 494 in 2012.

Drones are highly effective and most nations have given Washington at least tacit agreement to carry out the attacks.

Peter Singer of the Washington-based Brookings Institution noted that the drone program in Afghanistan is run by the Pentagon, and therefore is more transparent than the CIA drone counter-terrorism program in Pakistan.

Singer, who has written extensively about drones, said the number of operations in Afghanistan is increasing, but most are performed in support of troops on the ground.

"This is just another sign of how drones are becoming the new normal," he said.

The U.N. figures were released as part of its annual report on civilian casualties in Afghanistan. Overall, the full-year toll of civilian deaths in 2012 declined to 2,754, a 12 percent decrease from 3,131 in the same period a year earlier. It was the first time in six years that the civilian death toll dropped.

But the toll spiked in the second half of the year as weather improved, compared with the same period in 2011, suggesting that Afghanistan is likely to face continued violence as the Taliban and other militants fight for control following the impending withdrawal of U.S. and allied combat forces.

The population also faced a sharp increase in assassinations and other insurgent attacks targeting government supporters.

Conflict-related violence struck more women and girls last year as well, with 301 killed and 563 wounded a 20% increase from 2011, the report said.

The findings come as the war is reaching a turning point, with international troops increasingly taking the back seat in operations and Afghan government forces in the lead.

The total number of civilian deaths by airstrikes fell for the year after the US-led coalition implemented stricter measures to prevent innocent people from being killed.

The UN said most civilian casualties from drone strikes appeared to be the result of weapons aimed directly at insurgents, but some may have been targeting errors. It cited the example of four boys killed in October in Logar province when a drone struck after a clash between pro-government forces and insurgents a few kilometers (miles) away from the area.

Unama called for a review of tactical and operational policy on targeting to ensure compliance with international humanitarian law "with the expansion of the use of unmanned combat aerial vehicles" in Afghanistan.

George Little, a Pentagon spokesman, said the US works hard to protect civilians.

"We take great care with our unmanned systems to conduct very precise targeting in Afghanistan, and we will continue to do so. When there are mishaps, we take steps to work closely with the government of Afghanistan and the affected individuals to express our concerns," he said in Washington.

Unama said civilian casualties rose 13 percent to 4,431 in the second half of the year, including more from roadside bombs in public areas, compared with the same period in 2011.

That included 1,599 people killed and 2,832 wounded from July 1 to December 31, a jump from 1,556 and 2,832 respectively in the same period the previous year.

It cited a growing number in civilian casualties from roadside bombs even as fewer bystanders were hurt in ground engagements in the country's troubled south and east.

An Afghan interior ministry spokesman pledged to do everything possible to stop the insurgents from attacking civilians.

"They're still using suicide bombers, they still use IEDs (roadside bombs) in the very populated areas and they still use civilians as a shield in the villages," Interior ministry spokesman Sediq Sediqi said. "The important thing is that civilian casualties should be decreased to zero."

Most of the victims were killed by Taliban militants and other armed groups, while the number of civilian casualties at the hands of U.S. and allied forces dropped by nearly 50%, according to the report.

"The situation for civilians is still very difficult in many communities and many thousands of Afghans are still affected by the armed conflict, so we are again calling on all concerned to redouble their efforts, increase their efforts to protect civilians," Unama's Gagnon told reporters in Kabul.

The Unama report attributed the overall drop in civilian casualties for the year to a decline in suicide attacks, reduced numbers of airstrikes and "an unseasonably harsh winter which impeded insurgent movements and effects of earlier military operations against anti-government elements."

But it expressed concern about the spike in targeted killings and human rights abuses by armed groups, a worrisome trend as the Afghan government works to assert control beyond its seat in Kabul.

The Taliban and other insurgents were responsible for 81% of the civilian casualties last year, the U.N. said. The report said so-called anti-government elements killed 2,179 civilians and wounded 3,952, a 9 percent increase in casualties from 2011.

Of those, 698 were killed in targeted attacks, often against government employees. That was up from 512 in 2011.

The number blamed on US and allied forces, meanwhile, decreased by 46 percent, with 316 killed and 271 wounded in 2012. Most of those were killed in US and Nato airstrikes, although that number, too, dropped by nearly half last year to 126, including 51 children.

The death of civilians in military operations, particularly in airstrikes, has been among a major source of acrimony between Karzai's government and foreign forces.

The US-led military coalition said in June it would only use airstrikes as a self-defense weapon of last resort for troops and would avoid hitting structures that could house civilians.

The report came a day after Karzai banned government forces from requesting foreign air support during operations in residential areas amid anger over an airstrike that killed at least 10 civilians in northeastern Kunar province last week.