WASHINGTON (Reuters) - The United States strongly condemned the killing of up to 20 U.N. employees in an attack in northern Afghanistan on Friday, saying there was no justification for the murder of innocent people.

"I condemn in the strongest possible terms the attack on the United Nations Assistance Mission in Afghanistan today," President Barack Obama said in a statement. "Their work is essential to building a stronger Afghanistan for the benefit of all its citizens. We stress the importance of calm and urge all parties to reject violence."

Afghan protesters, angered by the burning of a Koran by an obscure U.S. pastor, killed up to 20 U.N. staff in Mazar-i-Sharif on Friday in the worst attack ever on the United Nations in Afghanistan.

The State Department said the United States stood ready to assist the United Nations in any possible way, and underscored that the United States believed the pastor's desecration of the Koran was "an abhorrent act."

"We've been very clear in saying that this is an isolated act done by a small group of people," State Department spokesman Mark Toner said.

"It doesn't reflect the respect that people of the United States have toward Islam and we absolutely reject this kind of intolerance," he said.

(Reporting by David Alexander and Andrew Quinn; Editing by Vicki Allen)