President Obama on Monday expressed “deep concern” about a spate of attacks on U.S. and NATO troops in Afghanistan by members of the Afghan government’s army and police forces, or assailants wearing their uniforms.

At least 28 so-called “insider attacks” have occurred this year, resulting in the deaths of 39 coalition troops, including 23 Americans. The killings, also known as “green-on-blue” attacks, continued Friday, when an Afghan Local Police officer fatally shot two U.S. troops during a training exercise in western Afghanistan’s Farah province.

Addressing the issue for the first time, Obama told a White House news conference Monday: “On Afghanistan, obviously we’ve been watching with deep concern these so-called green-on-blue attacks, where you have Afghan individuals, some of whom are actually enrolled in the Afghan military ... in some cases dressing up as Afghan military or police, attacking coalition forces, including our own troops.”

He said he spoke Monday to Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, who is “having intensive consultations” in Afghanistan now with the top U.S. commander there and with Afghan counterparts. “And I’ll be reaching out to President [Hamid] Karzai as well, because we’ve got to make sure that we’re on top of this,” Obama said.

He said U.S. forces already are “seeing some success when it comes to better counterintelligence, making sure that the vetting process for Afghan troops is stronger.” He cited a “Guardian Angel program” designed to ensure that U.S. troops do not find themselves “in isolated situations that might make them more vulnerable.” But he said that “obviously we’re going to have to do more because there has been an uptick over the last 12 months on this.”

Obama noted that in pursuing a plan to transfer more security responsibilities to Afghan forces, “our troops are in much closer contact with Afghan troops on an ongoing basis” in order to train them effectively. He said the military faces a challenge to “make sure that this model works” without further endangering U.S. troops.

“In the long term, we will see fewer U.S. casualties and coalition casualties by sticking to our transition plan and making sure that we’ve got the most effective Afghan security force possible,” Obama predicted. “But we’ve got to do it in a way that doesn’t leave our guys vulnerable.”

He added: “So we are deeply concerned about this from top to bottom. And hopefully over the next several weeks we’ll start seeing better progress on this front.”