United Nations chief Ban Ki-moon on Monday called the killing of al Qaeda leader Osama bin Laden a "watershed moment" in the war against terrorism, expressing relief that "justice" had been done.

"The death of Osama bin Laden, announced by President (Barack) Obama on Sunday night, is a watershed moment in our common global fight against terrorism," Ban said in a statement.

"The crimes of Al Qaeda touched most continents, bringing tragedy and loss of life to thousands of men, women and children," he added.

President Barack Obama early this morning announced that bin Laden had been killed by US forces, a decade after he masterminded the 9/11 terror attacks that claimed the lives of 3000 people.

"The death of bin Laden marks the most significant achievement to date in our nation's effort to defeat Al Qaeda," said Obama, in a televised speech. In his statement, Ban recalled that he was in New York on 9/11 and also said that justice had been done.

"I remember, personally, vividly, the day of September 11, 2001. I was in New York on that dark day," he said. "I am very much relieved by the news that justice has been done to such a mastermind of international terrorism," he said.