The file photo shows that U.S. soldiers inspect the site of suicide car bombing in Kabul, Afghanistan, May 17, 2015. (Xinhua/Ahmad Massoud)

WASHINGTON, June 10 (Xinhua) -- The U.S. Defence Department confirmed on Saturday that three U.S. soldiers were killed and another wounded in eastern Afghanistan.

The Pentagon announced the casualties in a briefing statement that did not detail the incident.

"The incident is under investigation, said the statement, adding more information will be released as appropriate."

However, according to local sources, the attack was an insider attack targeting U.S. soldiers by an Afghan solider.

"The incident took place in Achin district and the Afghan army commando involved in the firing was also shot dead after attacking the U.S. soldiers," Ataullah Khogyani, the provincial governor spokesman, told Xinhua.

Also Known as the "green-on-blue" attack, the insider attack, where an Afghan government solider or police fires on NATO coalition forces, is one of the major problems facing coalition forces there.

Three U.S. soldiers were wounded after being shot by an Afghan soldier in a base in Helmand province on March 19.

The latest insider attack also happened as U.S. President Donald Trump was reportedly mulling over increasing troop levels for Afghanistan.

Currently, there are about 8,400 U.S. troops and another 5,000 NATO forces in Afghanistan to train and assist Afghan forces against the Taliban and conduct counter-terrorism missions.