CHICAGO — President Obama vowed that Afghanistan "will not stand alone" as NATO forces shift security responsibility to Afghan forces in the months ahead .

As NATO leaders met this morning to chart the next step toward transitioning security responsibility to the Afghans, Obama said he was confident that they could bring the war to an end responsibly.

Members of the alliance agreed to a plan that would transition Afghan forces into the lead of security — with NATO troops playing an advisory role — by the middle of 2013.

"This will be another step toward Afghans taking full lead for their security as agreed to by 2014 when the ISAF [International Security Assistance Force] combat mission will end," Obama said.

NATO, in fact, began the slow transitioning of security control to the Afghans 18 months ago. In the areas where the security transfer has already occurred, Afghan forces have performed well, and there hasn't been a dramatic uptick in violence, said Douglas Lute, a senior adviser to the president on Afghanistan-Pakistan issues.

"What we have seen is that the Afghan security forces, both army and police, have actually surprised us a bit, and maybe surprised themselves a bit by their ability to handle this," Lute said. "Now, admittedly, as you might expect, the early transition areas were the ones more ripe for transition, and we're getting now into some tough areas. So the next year, six months to 12 months in the transition process will be telling. "

Afghan President Hamid Karzai recently announced a third group of provinces in Afghanistan where security forces will soon be transitioning into the lead — with NATO forces remaining behind in an advisory role. With the move, 75% of Afghan people now live in areas where Afghan forces are in the lead.

The administration is pressing cash-strapped European governments to back Afghanistan's security over the next decade. The U.S. wants allies, many of which are cutting budgets, to help cover the $4.1 billion a year needed to finance Afghan security forces after 2014.

Lute said there was "substantial" progress toward the goal, but he did not disclose a dollar figure.

"We have a lot of work to do," Obama told world leaders at the summit. "Our nations and the world have a vital interest in the success of this mission. And I am confident, because of the leadership represented here as well as the leadership of our outstanding armed forces, that we can advance that goal today and responsibly bring this war to an end."