"I condemn in the strongest possible terms the attack on the United Nations Assistance Mission in Afghanistan (UNAMA).
Together with the American people, I offer my deepest condolences to those injured and killed, as well as to their loved ones," he said after an armed mob attacked the UN office in Mazar-i-Sharif during a demonstration against the burning of Quran by a US pastor.
"The brave men and women of the United Nations, including the Afghan staff, undertake their work in support of the Afghan people. Their work is essential to building a stronger Afghanistan for the benefit of all its citizens," Obama said.
"We stress the importance of calm and urge all parties to reject violence and resolve differences through dialogue," said the US President.
On behalf of the United States, Obama offered his deepest condolences to the families of the victims -- Special Representative Staffan de Mistura and his UNAMA colleagues.
In another statement, Secretary of State Hillary Clinton said the UN staff killed yesterday was in Afghanistan to help the people of the war-torn country build a better future.
"The United Nations has stood by the Afghan people for more than 50 years. They have saved countless lives and delivered essential food and supplies. They are a force for peace, progress and stability in Afghanistan and throughout the world," she said.
State Department spokesman Mark Toner said it is premature to speculate on the exact cause of the incident or linking it with the Quran burning incident in the US.
"About the burning of the Quran, we`ve been very clear in saying that this is an isolated act done by a small group of people, and it`s indeed very contrary to the American people`s traditions. It doesn`t reflect the respect that the people of the United States have toward Islam, and we absolutely reject this kind of religious intolerance," Toner said in response to a question.
White House Press Secretary Jay Carney also refrained from commenting on what may have been the cause of the incident which, he said, "is in no way justified, no matter -- regardless of what the motivation was."
"On the cause, the motivation behind it, I don`t want to speculate because it`s important that we get it right. So we are looking into that, investigating it," he said in response to a question.