By Mike Mount

For the past month, the U.S. military has experienced something not seen for five years in Afghanistan: No combat deaths.

Three U.S. troops have died from hostile fire injuries since Jan. 1, and one of them succumbed to wounds sustained in December.

The trend marks the longest period without a U.S. combat death in America's longest war since 2008, and clearly reflects a strategy shift that leaves much of the fighting to Afghan security forces, whose deaths are going up.

Afghans now lead more than 80% of combat operations and control areas covering more than three-quarters of the population, according to U.S. military officials.

The U.S. military has pulled back from direct combat operations into the less dangerous role of advising and assisting Afghan forces.

American military officials said a cut in the number of American forces is another reason for the decline.

There were about 100,000 forces in Afghanistan during the peak of the military's troop surge. But that number fell by almost 40 percent when the last of those troops left in September and remains at about that level today.

Attacks by Taliban insurgents also have declined, officials have said.

Just as the U.S. toll has dropped, Afghan security force deaths have risen sharply.

"The Taliban are targeting the Afghan Army and police to try and show the populous the Afghan Security Forces cannot adequately protect them," said Col. David Lapan, spokesman for the chairman of the Joint Chiefs of Staff.

"They are trying to undermine the future of the (Afghan Security Forces)," he said.

Having reached their peak of 352,000 last year, more Afghan security forces will also bring more casualties as well, according to defense officials.

"They really have taken on the bulk of the security responsibility and have been dying in very significant numbers as compared to their U.S. counterparts over the last three months," said Jeffrey Dressler, a senior analyst on Afghanistan with the Washington-based institute for the Study of War.

Last year was the deadliest so far for Afghan forces with more than 3,400 soldiers and police killed, up from 1,950 the previous year, according to a Brookings Institute study.

Some 237 U.S. troops were killed by hostile fire in 2012, down from 351 the previous year, according to Pentagon statistics.

While Afghan security force fatalities are up, a new report by the United Nations on Tuesday shows civilian deaths on the decline.

Civilian deaths in Afghanistan dropped 12% in 2012 - the first time that figure has fallen in six years, according to the U.N. Assistance Mission to Afghanistan (UNAMA).

The report credits the decline to fewer suicide bombings, a drop in aerial attacks and less ground fighting between pro-government and militant forces.

The U.S.-led NATO military force has put strict limits on air strikes by coalition aircraft called in by NATO troops.

Last week, Afghan President Hamid Karzai banned Afghan security forces from calling in allied air strikes after at least 10 civilians were killed during an Afghan-led operation in eastern Kunar province earlier this month.