Two U.S. soldiers killed at base in Afghanistan

* Incident was the second in less than a week

* Pentagon disputes suggestions Afghanistan strategy failing

* Afghan officials worry about damaged ties with NATO (Adds White House comment)

By Michael Georgy

KABUL, March 1 (Reuters) - Two U.S. soldiers were shot and killed on Thursday in an attack involving at least one Afghan believed to be a soldier and a civilian, Western and Afghan officials said, the second such incident in a week and one likely to deepen doubts about Afghanistan's security forces.

The killings at a base in Kandahar in southern Afghanistan came less than a week after two senior U.S. officers were gunned down in the Afghan Interior Ministry on Saturday by what Afghan security officials say was a police intelligence official.

Western officials have been hoping the violent outcry that erupted last week in Afghanistan following the burning of copies of the Koran at a NATO military base will soon pass, allowing them to focus on the other major challenges that remain before the end of 2014, when most foreign combat troops are due to leave Afghanistan.

Western and Afghan officials initially said Thursday's attack had been carried out by one man in an Afghan military uniform and a second in civilian clothes.

But the Pentagon said later it appeared three Afghans were involved, including two soldiers who were killed by members of the International Security Assistance Force and a civilian who may have been an instructor at the base. It was not immediately clear whether the civilian had escaped or not.

Pentagon Press Secretary George Little said the motive for the attack was not initially clear. About 70 members of the NATO-led force were killed in 42 insider attacks from May 2007 through the end of January this year.

Some of these incidents have been carried out by Afghan security forces reacting to the recent Koran burning, some have been due to private grievances and others have been carried out by the Taliban insurgency.

The killing of the U.S. officers in the Interior Ministry on Saturday stunned NATO and cast doubt on its strategy of replacing large combat units with advisers as the alliance tries to wind down the war, now in its 11th year.

NATO immediately moved to withdraw all its advisers from Afghan ministries in Kabul. Britain, Germany and Canada then withdrew their advisers.

Some NATO staff have been allowed to go back to the ministries, said Brigadier General Carsten Jacobson, the spokesman for NATO's International Security Assistance Force.

On Wednesday, President Barack Obama said that while the situation remained volatile, his formal apology to Afghan President Hamid Karzai over the Koran burnings had "calmed things down" in Afghanistan.

The White House, following the two soldiers' death on Thursday, sought to head off any speculation that Obama has suggested violence had disappeared entirely the day before.

"Nobody has suggested that violence has ended in Afghanistan, in general or in reaction to the unfortunate incident involving the inadvertent, unintentional burning of religious material," White House spokesman Jay Carney told reporters.

Obama, who is running for re-election in November, is seeking to deflect criticism by Republicans for making what they call unnecessary apologies for U.S. actions overseas. At the same time, he does not want to be seen as playing down U.S. casualties in a costly, unpopular war he hopes to soon end.

"STAYING THE COURSE"

Little disputed suggestions that the recent killings mean the alliance's strategy is not working. He said that Afghan soldiers themselves have been killed in the past week while trying to tamp down the violence provoked by the Koran burnings at an ISAF base where detainees are held.

"I strongly reject the notion that our strategy in Afghanistan is failing," he said. "There have been suggestions this week that the wheels on the bus are falling off and that is simply not the case."

When viewed broadly "the overall trends are positive," Little said, adding, "We are staying the course in Afghanistan."

Despite such assurances, Afghan officials worry that further attacks by Afghan forces on Western troops could damage ties with NATO.

Such incidents became more frequent after the United States sent tens of thousands of more soldiers to Afghanistan as part of a surge to fight in Taliban strongholds.

"There are Taliban sympathisers in uniform inside Afghan security forces who are not in fact sent or recruited by the Taliban," said an Afghan government official, who spoke on condition of anonymity.

"Despite tighter vetting procedures, such unfortunate incidents do occur. This problem will not go away. We need more time, more resources and manpower."

Some of Washington's partners have shown even greater sensitivity to insider attacks. In January, French President Nicolas Sarkozy suspended training and support operations and announced that France would withdraw entirely by the end of 2013 after four French troops were killed by a rogue Afghan soldier.

The United States hopes Afghan forces will be able to confront the Taliban and handle security on their own before NATO combat troops' scheduled departure by the end of 2014.

"Unfortunately, this situation is a point of concern for us," General Afzal Aman, head of the operations department at the Ministry of Defence, told Reuters, referring to the insider killings of NATO troops. (Additional reporting by Mirwais Harooni and Hamid Shalizi in Afghanistan and David Alexander, Alister Bull and Missy Ryan in Washington, Editing by Robert Birsel and Xavier Briand)