WASHINGTON: President Barack Obama said Monday the United States was watching with “deep concern” an increase in so-called “green on blue” attacks on US troops in Afghanistan by insiders in Afghan uniform.

“Obviously, we have to do more, because there is an uptick over the last 12 months or so on this,” he said at a White House news conference.

Obama emphasized that Afghan security forces must be trained to take over from withdrawing US and Nato troops.

“In the long-term, we will see fewer US casualties and coalition casualties by sticking to our transition plan and making sure that we have the most effective Afghan security force possible, but we have to do it in a way that doesn't leave our guys vulnerable,” he said.

His comments follow a spate of deadly attacks on US and Nato troops by Afghan troops who have opened fire on their foreign trainers.

“Obviously, we have been watching with deep concern the so-called green-on-blue attacks where you have Afghan individuals, some of whom are actually enrolled in the Afghan military and in some cases dressing up as Afghan military or police, attacking the coalition's, including our own troops,” Obama said.