US President Barack Obama welcomed the Kabul government's move Sunday to have local forces take control of security in a large new slice of the country as an "important step forward."

In the third phase of a five-tranche military transition process to bring NATO closer to getting out of the Afghan war, 122 more districts throughout Afghanistan will come under local command, putting Afghan forces in control of security for 75 percent of the population.

"I welcome President (Hamid) Karzai's announcement today of the third tranche of areas to transition to Afghan security lead, which is an important step forward in our effort to achieve our objectives in Afghanistan," Obama said in a statement.

US Defense Secretary Leon Panetta said he was "very pleased" with the development, calling it "another tangible sign of progress in our strategy and in the further development of Afghan leadership and responsibility for their own security."

Praising the "growing strength" of Afghan security forces, he stressed that the phase was the largest yet, and that when implemented, about three fourths of Afghans will live in areas transitioning to Afghan security control.

Afghan government officials said the handover's third phase would start immediately and could take as little as six months, although according to NATO's International Security Assistance Force complete transition in an area can take 12-18 months.

NATO has a total of 130,000 soldiers helping the Karzai government fight an insurgency by hardline Taliban militants, and they are due to withdraw by the end of 2014 when the transition process is complete.

Obama, who is set to host a NATO summit in his adopted hometown of Chicago on May 20-21, said Afghan National Security Forces were "strengthening their capacity," allowing the security transition to remain "on track."

While NATO is quick to praise the growing skills of the 350,000-strong Afghan security forces it is training to take over, officers acknowledge the war will not be won on the battlefield.

The White House has said the United States will push to modernize the North Atlantic Treaty Organization, deepen partnerships and hammer out details of the Afghanistan withdrawal at the summit.

"The Afghan National Security Forces are strengthening their capacity as we remain on track to meet our goal of having the Afghan government fully responsible for security across the country by the end of 2014," Obama said.

"I look forward to meeting with President Karzai and my fellow leaders in Chicago to discuss these critical steps that will strengthen Afghan sovereignty while responsibly winding down the war."

He said world leaders would discuss how to "effectively advance" the transition process.

At the summit, NATO will be faced with the thorny issue posed by French president-elect Francois Hollande's pledge to speed up his country's pullout from Afghanistan.

The French Socialist leader campaigned on a promise to start bringing 3,300 French soldiers home this year, ending his country's combat role two years earlier than NATO's carefully crafted plan to fully hand security control to Afghans.