2 US soldiers killed amid mob violence across Afghanistan (VIDEO)

Two U.S. soldiers have been shot to death by a man in an Afghan border police uniform amid protests in Afghanistan against the burning of a Quran by a fundamentalist American pastor Terry Jones.

The gunman fled after shooting the Americans, who were attached to the NATO-led International Security Assistance Force (ISAF) and helping to train members of the country's security forces in Faryab province in the north, the Associated Press reported.

The ISAF said it was investigating the shooting, which it said was carried out by "an individual in an Afghan Border Police uniform." It is the latest in a series of shootings in which a member of the Afghan security forces has turned a weapon on Western trainers tasked with transforming the Afghan police and army into a fighting force, according to the LA Times.

Meanwhile, the top U.S. commander in the region, Gen. David Petraeus, has warned that the burning of a Quran at a church in Florida on March 20, which sparked four days of deadly rioting in Afghanistan, poses new dangers for the U.S.-led war effort against the Taliban.

The deadly rioting, which the Taliban say erupted spontaneously, according to the Wall Street Journal, has shocked the international community. Anti-Western sentiment has been also running high because of civilian causalities from NATO drone strikes.

In the deadliest incident so far, seven U.N. staff were among 14 people killed on Friday, along with five Afghans, after demonstrators overran their office in the normally-peaceful city of Mazar-e-Sharif in the country's north.

And 12 people died in Kandahar over the weekend, when demonstrators waving Taliban flags and shouting "death to America" burned cars, smashed shops and sacked a high school for girls.

Protesters turned out across Afghanistan for a fourth day Monday, with rock-throwing crowds hundreds deep clashing with Afghan police in several places in the east.

In the eastern city of Jalalabad, up to 1,000 protesters blocked the main highway to Kabul and set alight effigies of the pastor, Terry Jones, the Gainesville-based pastor who presided over the Quran burning in Florida.

Demonstrations also took place in Lashkar Gah, the capital of Helmand province — one of seven areas designated by President Hamid Karzai where Afghan police and army are to take over from the ISAF this year. More protests took place in Laghman and Paktia provinces, in eastern Afghanistan.

Urban mob violence against Western targets adds a disturbing new threat in a country where security forces are ill-prepared for riot control, Petraeus said in an interview.

"Every security force leader's worst nightmare is being confronted by essentially a mob, if you will, especially one that can be influenced by individuals that want to incite violence, who want to try to hijack passions, in this case, perhaps understandable passions," he said. "Obviously it's an additional serious security challenge in a country that faces considerable security challenges."

The White House on Monday slammed Quran burning as "un-American" but said the destruction of the Muslim holy book did not justify the killings of U.N. personnel, Agence France-Presse reported.