A growing number of attacks by Afghan troops on U.S. and NATO forces is raising questions as Afghanistan prepares to take more responsibility for security.

NATO said Wednesday a man wearing an Afghan army uniform shot and killed a coalition service member in southern Afghanistan, the same day U.S. lawmakers demanded better screening to prevent so-called insider attacks.

Afghan officials said the latest shooting took place in the Marjah district of Helmand province Tuesday, and the Afghan soldier claimed the death was an accident. But statistics shows similar attacks have increased dramatically in recent years.

The U.S. Defense Department says of the more than 40 attacks by Afghan security personnel on American troops since 2007, 75 percent have taken place in just the past two years.

The chairman of the House of Representatives' Armed Service Committee says the vetting of Afghan security forces “has been tragically weak.”

Republican Representative Howard McKeon called for U.S. Defense officials to do more to screen out troops, police or Afghan security contractors who could pose a threat to American personnel. He also accused Afghan President Hamid Karzai of complicating the situation by banning the use of foreign security contractors in Afghanistan.

U.S. Army Brigadier General Stephen Townsend agreed more needs to be done, especially as U.S. troops will increasingly be embedded with Afghan forces as trainers and advisors. He said the U.S. military is increasing its use of biometric technology to prevent Afghans who could pose a threat from working with U.S. troops.

Deputy Assistant Defense Secretary David Sedney told the committee part of the reason for the jump in the number of attacks is the increasing success U.S. forces. He says that has made carrying out so-called “insider attacks” a higher priority for the Taliban and other insurgent groups.

U.S. data shows at least 60 percent of the attacks are motivated by personal issues.

U.S. officials say they are increasing the amount of training U.S. forces get to help them identify such personal issues – such as insults or slights – that could result in an “insider attack.” They also say Afghan forces have started to train their security personnel on cultural differences that could cause friction.

Last week, France announced it was pulling its combat troops from Afghanistan a year early, in 2013, after an Afghan soldier killed four French troops during a training exercise in eastern Afghanistan on January 20.

Earlier this month, an Afghan soldier shot and killed an American service member in the south. And in December, an Afghan soldier opened fire and killed two members of the French Foreign Legion.

NATO officials have said such incidents are isolated cases.