Hours after President Obama claimed his apology for Quran burnings had "calmed things down" in Afghanistan, two more American soldiers were killed by their Afghan colleagues Thursday. Officials said the Americans were shot to death at a joint U.S.-Afghan base in southern Afghanistan. The killers were two Afghan soldiers and a civilian literacy teacher who fired from a sentry tower at the base in Kandahar province's Zhari district, a Taliban stronghold.

The slayings brought to six the number American soldiers killed since the Feb. 20 burning of Qurans and other Islamic texts at Bagram Airfield near Kabul. An Afghan soldier opened fire during Feb. 23 riots in eastern Afghanistan, killing two U.S. soldiers. Two days later, a gunman killed an American colonel and major inside Afghanistan's Interior Ministry. More than 30 Afghans have died in the violence that followed the Quran burnings.

Following the Feb. 25 murders, Gen. John Allen, commander of U.S. forces in Afghanistan, ordered all military advisors with NATO's International Security Assistance Force (ISAF) to withdraw from government ministries for safety reasons. Thursday's killings occurred on the same day that NATO permitted a small number of advisors to return to work at Afghan ministries.

The ISAF has stepped up training of Afghan security forces to enable them to take the lead in fighting al-Qaida and the Taliban and permit NATO to withdraw. But efforts to forge a U.S.-Afghan partnership are being undermined by the rising number of attacks on Western forces by Afghan police and soldiers (or jihadists who have managed to infiltrate).

Meanwhile, Afghanistan Defense Minister Abdul Rahim Wardak warns that a U.S. proposal to slash the size the size of Afghan security forces from 352,000 this year to less than 230,000 after 2014 could lead to catastrophe.

"Nobody at this moment, based on any type of analysis, can predict what will be the security situation in 2014. That's unpredictable," Wardak said. "Going lower [in Afghan troop numbers] has to be based on realities on the ground. Otherwise, it will be a disaster; it will be a catastrophe."

According to the Long War Journal, U.S. troop withdrawal plans are being driven more by a desire to cut costs than the Afghan forces' ability to do their job properly. The new commander of the NATO training mission, U.S. Army Lt. Gen. Daniel Bolger, said the proposal for a smaller U.S. force reflects "our assessment of what the international community will provide and what the Afghans can provide for themselves."

Washington has yet to spell out what happens if Kabul and the international community aren't up to the task.