The transition, which began in early 2011, is slated to give Afghan forces full responsibility for security by the end of 2014, when most NATO troops will have withdrawn.
Misgivings persist about the readiness of Afghan forces, although their numbers have grown rapidly over the past year to more than 330,000. They now shoulder most combat operations, while NATO forces, including some 66,000 U.S., troops are preparing to pull out. The U.S. intends to keep a residual force in Afghanistan past 2014 but the size has yet to be determined.
U.S. Defense Secretary Leon Panetta said in a statement said the forthcoming fourth phase was "another sign of steady progress."
"Going forward, our efforts in Afghanistan will continue to ensure that the Afghan people can secure and government themselves, and to deny safe haven to al-Qaeda," the statement said.
Ashraf Ghani Ahmadzai, who heads a transition commission, told a news conference that Afghan security forces now are responsible for protecting 75 percent of the population.
"The general assessment is that security is better or the same," Ahmadzai said, referring to changes over the third transition phase, which began in May and ended Monday. He said that by the end of the fourth phase, the duration of which is open-ended, 87 percent of the people will be protected by Afghan forces.
Targeted for the upcoming transition are 12 provinces mostly in the north and central regions as well as a district in the southern province of Helmand, the most violent in the country.
Gen. John R. Allen, commander of foreign forces in the country, called the announcement "another historic step as (Afghanistan) gets closer to taking full responsibility for security of the entire country."
NATO Secretary General Anders Fogh said in a statement that it was a "significant step toward our shared goal of seeing Afghans fully in charge of their own security by the end of 2014."