An American soldier was killed Wednesday in a firefight between US and Afghan troops in eastern Afghanistan, the first apparent insider attack since Washington announced a delay in troop withdrawals from the country.

So-called "green-on-blue" attacks -- when Afghan soldiers or police turn their guns on international troops -- have been a major problem during NATO's long years fighting alongside Afghan forces.

Wednesday's attack marked the second such fatal incident since the start of the year, highlighting long-simmering tensions between Afghan and foreign forces.

The gunfight erupted after an Afghan soldier opened fire on an American delegation outside the governor's compound in Jalalabad, the capital of the militant-infested province of Nangarhar.

"US soldiers returned fire, killing the shooter and wounding two other members of the Afghan National Army. We are investigating to find out the motive behind the shooting," Nangarhar police chief Fazel Ahmad Shirzad told AFP.

An American soldier, who was part of a unit devoted to training and advising Afghan forces, was also killed, US officials told AFP.

The "incident in Jalalabad today resulted in the death of one Resolute Support service member", said a brief statement from Resolute Support, the new name for the NATO mission in Afghanistan.

Monica Cummings, the US embassy spokeswoman, said the incident occurred after a senior US official held a meeting with the provincial governor in Jalalabad.

"All Chief of Mission personnel of the visiting party are accounted for," she added in a statement, without offering any more details.

- Fierce mistrust -

The Taliban did not immediately claim responsibility for the attack, and Western officials say that most such attacks stem from personal grudges and cultural misunderstandings rather than insurgent plots.

The killings have bred fierce mistrust among local and foreign forces even as the rate of such incidents has dropped in recent years.

Most NATO combat troops pulled out of Afghanistan last year but a small contingent remains, including roughly 10,000 American soldiers.

The US soldiers, along with other NATO troops and private contractors, are focusing on training Afghanistan's national security forces.

One of the worst insider attacks took place last August when US Major General Harold Greene was killed -- the most senior American military officer to die in action overseas since the Vietnam War.

Three American contractors were killed in an apparent insider attack at Kabul airport in January.

NATO troops have adopted special security measures in recent years to try to counter the threat.

The Afghan military, which has been built from scratch since the fall of the Taliban regime in 2001, has also struggled with "insider attack" killings, high casualty rates and mass desertions.

President Barack Obama last month reversed plans to shrink the US force in Afghanistan this year by nearly half, an overture to the country's new reform-minded leader, President Ashraf Ghani.

Hosting Ghani at the White House for their first presidential head-to-head, Obama agreed to keep the current level of 9,800 US troops until the end of 2015.

The Taliban, waging a deadly insurgency since they were ousted from power in late 2001, warned that the announcement would damage any prospects of peace talks as they vowed to continue fighting.

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