The top U.N. official in Burma says 22,000 people have been displaced in a week of sectarian fighting between Rakhine Buddhists and Rohingya Muslims in Burma's Rakhine state.

The U.N. chief in Rangoon, Ashok Nigam, said Sunday that getting aid to those who fled their homes will be a challenge, because some fled on boats and others have sought refuge on isolated hilltops.

The Burmese government said Saturday that the fighting has left more than 2,800 houses burned down and 67 people dead in the past week.

New York-based Human Rights Watch said in a statement Saturday that it fears the death toll is far higher, based on the accounts of witnesses fleeing the fighting, which it says has disproportionately targeted Muslims.

On Thursday, the U.N. released a statement calling on Burmese authorities to bring lawlessness and vigilante attacks under control and to put a stop to threats and extremist rhetoric.

Human Rights Watch said that in June it documented killings, rape and mass arrests by Burmese security forces against Rohingya Muslims after troops failed to protect both them and Arakan Buddhists during deadly sectarian violence.

Some rights groups say the Burmese military, which has a long history of abusing minorities, unfairly targeted Muslims during the unrest. Burma's government denies the charges.