The number of U.S. military members killed in the war in Afghanistan has surpassed 2,000, with more than half of the deaths coming in the past 27 months.

The increased casualty rate came as President Barack Obama poured thousands of extra troops into the country to intensify the U.S.-led NATO coalition's battle against insurgents.

Those troops are withdrawing, along with their international counterparts, as Afghan forces take increasing security control ahead of a 2014 deadline for foreign combat troops to leave Afghanistan.

The Afghan army and police have endured more casualties since the war began in 2001. Statistics dating back to 2007 show more than 6,500 Afghan security force members have been killed.

The United Nations has reported that more than 13,000 Afghan civilians have also died as a result of the conflict since 2007.

U.S. military officials have expressed concern about recent attacks by members of the Afghan security forces on international troops that have killed 10 of them, mostly Americans. At least 39 coalition members have been killed in such attacks this year.

On Wednesday, Afghan President Hamid Karzai's office said he met with his national security team to discuss the problem. It cited measures being taken against “foreign infiltration” of Afghan forces, but named no countries. Afghan officials have in the past blamed Iran and Pakistan for supporting the insurgency.

President Karzai's office said he directed authorities to take extra steps “in adding institutional strength” to the army, police and intelligence agency.

Joint Chiefs of Staff chairman General Martin Dempsey was in Afghanistan earlier this week for talks with NATO and Afghan commanders that focused on the insider attacks.

Afghan militants on Tuesday fired rockets at a plane used by the top U.S. military officer, but a spokeswoman told VOA he was “nowhere near” the aircraft at the time. The Taliban claimed responsibility for the attack.