WASHINGTON The American ambassador to Afghanistan vowed Sunday to "redouble" efforts despite an insidious Taliban attack at the interior ministry in Kabul that killed two US military officers.

NATO pulled all its advisors out of government ministries after the shootings on Saturday, which were blamed on a rogue Afghan intelligence official and claimed by the Taliban as a response to a recent Koran burning incident at a US base.

"Tensions are running very high here and I think we need to let things calm down, return to a more normal atmosphere, and then get on with business," said Ambassador Ryan Crocker, on a sixth day of violent anti-American protests.

"This is not the time to decide that we're done here. We have got to redouble our efforts. We've got to create a situation in which Al-Qaeda is not coming back," he told CNN's "State of the Union" program.

The two slain American officers, working as NATO advisors, were in the interior ministry on Saturday when "an individual" turned his weapon against the pair, the military alliance said, without giving further details.

The Taliban claimed responsibility for the shootings, saying it was in revenge for the burning of the Korans -- an incident that forced President Barrack Obama to apologize to the Afghan people.

The toll since the Koran burning incident at the Bagram airbase north of Kabul, which inflamed anti-Western sentiment already smoldering in Afghanistan over abuses by US-led foreign troops, rose Sunday to more than 30.

Senator John McCain, also speaking to CNN, said he understood the "anger and frustration and sorrow" that the American people feel at the "terribly unfortunate situation" in Afghanistan but urged them to stay the course.

"Have no doubt, that if Afghanistan reverts to a chaotic situation, you will see Al-Qaeda come back and it again be a base eventually of attacks on the United States of America," warned McCain, a celebrated Vietnam veteran who lost out to Obama in the 2008 presidential race.

A US-led coalition invaded Afghanistan after the September 11, 2001 attacks on the United States that left almost 3,000 people dead, hoping to ensure that Al-Qaeda would never again have safe haven to plot such destruction.

Nearly 90,000 US troops remain deployed in Afghanistan, propping up the government of Western-backed President Hamid Karzai. There are plans for the force to decline to 68,000 by the end of September.

Top Afghan officials and American commanders have suggested the United States will likely retain a military presence in Afghanistan after 2014, when Afghan army and police are due to take over security for the whole country.

Karzai has repeatedly invited the Taliban for direct talks with his government, urging neighboring Pakistan -- where many insurgents hide out in the rugged border areas -- to help facilitate negotiation efforts.