The White House on Thursday defended the apology rendered by U.S. President Barack Obama over the burning of Quran by U.S. troops in Afghanistan, saying it was "wholly appropriate" because his primary concern was the safety of Americans.

"It is wholly appropriate, given the sensitivities to this issue, the understandable sensitivities," White House spokesman Jay Carney told reporters aboard Air Force One as he was accompanying Obama to Miami.

"His primary concern as Commander-in-Chief is the safety of American men and women in Afghanistan, of our military and civilian personnel there," he said. "It was absolutely the right thing to do."

When asked if Obama was worried that it would feed the Republicans' ammunition to attack him for being weak on foreign policy, Carney said: "That's a fully false, fallacious and ridiculous narrative that is not borne out by any facts."

He also stressed that the letter, which contained Obama's apology and was sent to Afghan President Hamid Karzai, was "a lengthy three-page letter on a host of issues, several sentences of which relate to this matter."

Protests in Afghanistan over the alleged Quran burning by U.S. troops in Bagram airbase entered its third day on Thursday, leaving 15 people dead and 59 others wounded across the country.

With violence that is getting out of control, U.S. President Barack Obama was forced to render his apology for the incident. A statement released by the Afghan Presidential Palace on Thursday said: "In the official letter conveyed to the President this afternoon by Ryan Crocker, the U.S. ambassador to Afghanistan, President Obama has written that the incident in their facility was not intentional and assured the President of full investigation."

According to the statement, Obama wrote, "I wish to express my deep regret for the reported incident. I extend to you and the Afghan people my sincere apologies."

"The error was inadvertent; I assure you that we will take the appropriate steps to avoid any recurrence, to include holding accountable those responsible" the letter concludes, according to the statement.