The United Nations has expressed concern about the fate of civilians caught in the U.S.-led siege on the Iraqi flashpoint city of Fallujah.

The U.N. refugee agency said Tuesday it is concerned about the tens of thousands of people believed to have fled the city ahead of the assault. U.S. officials say at least half of the city's residents have evacuated, but as many as 100,000 remain.

The refugee agency has no presence on the ground in Iraq, but it said its local sources indicate many residents are staying in homes and tents near the city.

Iraq's government says there is no humanitarian crisis at this time, but that officials and their coalition allies are prepared to assist in any humanitarian situation that may arise.

Some of information for this report provided by AFP.