President Barack Obama said Wednesday he was "confident" the United States could stick to its Afghan drawdown timetable despite a week of deadly unrest over the burning of the Koran at a US base.

"I feel confident that we can stay on a path that by the end of 2014, our troops will be out and will not be in a combat role and Afghans will have capacity, just as Iraqis, to secure their own country," Obama told ABC News.

Obama, criticized by Republican opponents for apologizing to Afghan people after US troops sent copies of the Koran to an incinerator at Bagram airbase, defended his decision, saying it was necessary to try to quell the violence.

"The reason that it was important is the same reason that the commander on the ground, General (John) Allen, apologized. And that is to save lives. And to make sure our troops who are there right now are not placed in further danger," the president said.

"It calmed things down. We're not out of the woods yet," he added.

The incident set off seven successive days of protest and violence, with the death toll estimated at about 40.

Two US military advisers were gunned down in the interior ministry in Kabul on Saturday, days after two US troops were killed by an Afghan soldier in the east, prompting NATO to pull its advisors out of Afghan government ministries.

NATO has a 130,000-strong US-led military force fighting the Taliban, which has led an insurgency against the Western-backed Kabul government since being toppled from power in 2001.

The United States plans to gradually draw down combat troops from mid next year before handing over control to Afghan security forces by the end of 2014 as agreed by the NATO alliance.

Some Republican critics of Obama said the turmoil showed the need to slow the pace of the US drawdown, while opponents of the war saw it as more evidence of a doomed endeavor.