Osama bin Laden, the leader of al-Qaeda and once the most-wanted fugitive on the US list, has been killed in a firefight with American forces in Pakistan and quickly buried at sea, after a decade on the run.

Long believed to be hiding in the mountainous tribal region between Pakistan and Afghanistan, Bin Laden was tracked down in a custom-built mansion in the Pakistani town of Abbottabad, 61km north of the capital, Islamabad.

Confronted by elite US forces who dropped by helicopter into the compound, bin Laden was shot dead in a 40-minute operation under the cover of night, US officials said.

"Justice has been done," Barack Obama, the US president, announced in a televised address late on Sunday, sparking jubilant celebrations across the US.

"A small team of Americans carried out the operation with extraordinary courage and capability. After a firefight, they killed Osama bin Laden and took custody of his body.