(AFP/Getty Images)

A U.S. service member was killed in southern Afghanistan today on a patrol with Afghan security forces when one of the Afghans turned his weapon on the Americans, the latest in a rash of what the ISAF is now calling "insider" attacks on American troops.

The incident comes just days after the U.S. military told troops to be constantly armed and ready for attack, both inside and outside their bases. Twenty U.S. troops and one aid worker have been killed in Taliban attacks in the past two weeks, 10 of them shot to death in cold blood by rogue Afghan soldiers or policemen.

The number of "green on blue" attacks this year already exceeds the total for last year. Since the beginning of 2012, there have been 32 attacks resulting in 40 deaths, far more than last year's 21 total attacks.

A senior U.S. military officer at International Security Assistance Force headquarters in Kabul told ABC's Martha Raddatz forces throughout Afghanistan are now required to carry a loaded magazine in their personal weapon.

"Before, we usually did that only when we went out the gate," the official told Raddatz. "[Now] it applies everywhere. I am sitting in my office with a mag in my pistol. A round is not chambered, but I am ready to go at a moment's notice."