(ADEK BERRY/AFP/GettyImages)

A recent spate of deadly attacks by Afghan forces on U.S. troops has prompted the U.S. military in Afghanistan to change its policy on arming its members, ordering that service members now be armed at all times, inside and outside their bases.

In the most recent "green on blue" attack, an Afghan policeman turned his weapon on U.S. troops Friday, killing two soldiers.

This is just the latest in a deadly two-week span for U.S. forces, ABC's Muhammad Lila reports. Nineteen U.S. troops and one aid worker have been killed in Taliban attacks in the past two weeks, nine 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 31 attacks resulting in 39 deaths, far more than last year's 21 total attacks.

U.S. military and Afghan police will continue to work side by side as control of the country exchanges hands. In order to counter recent attacks, U.S. military leadership has altered its previous policy on when and where troops should be armed.

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."

The "green on blue" attacks might be shaking the trust between Afghan and ISAF partners in Afghanistan.

Earlier this year, Gen. John Allen, commander of all NATO troops in Afghanistan, counseled young Marines on how to react after these types of attacks.

"This is the time for professionalism," Allen said. "This is the time for trust and confidence, and this is not the time for revenge."

Marines on the ground told Raddatz earlier this year that they trust their Afghan partners, but the attacks have gotten worse since then.

The Taliban often claim responsibility for the attacks, and a recent Taliban-produced video showed an Afghan soldier receiving a hero's welcome after killing two U.S. soldiers.