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

The increase was a sign that unmanned aircraft 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 U.S. 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 airstrikes.

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 U.S. forces become more dependent on such attacks to fight al-Qaida and other insurgents.