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Four US soldiers killed in southern Afghanistan

Four U.S. soldiers were killed in an attack in southern Afghanistan on defense officials said Sunday.

The International Security Assistance Force did not identify the nationalities but U.S. defense officials said the four were Americans killed by a bomb in the early hours of Sunday.

Also in southern Afghanistan, a security guard shot a soldier dead in a separate incident, Reuters reported. It wasn't clear what the nationalities of the guard and the soldier were.

Reuters said that if the gunman turns out to have been an Afghan security guard for coalition troops, it would be the ninth insider attack this year, bringing the death toll of foreign personnel to 14.

That is down dramatically from last year, when a spate of so-called insider attacks prompted NATO to curtail some joint operations with Afghan government forces.

Last month, an Afghan believed to be a soldier killed three Americans in Gardez.

NBC News' Courtney Kube contributed to this report.