US snipers will take up positions on rooftops and local traffic will be barred from some downtown districts as part of unprecedented security precautions during US President George W. Bush's visit this week, Palestinian security officials said Tuesday.

Bush is expected to travel to Palestinian President Mahmoud Abbas' Ramallah headquarters, known in Arabic as the muqaata, by helicopter on Thursday.

Two alternate landing sites have been prepared; one in Abbas' compound and one in the parking lot of the nearby Quaker-run Friends' School. On Monday, helicopters practiced landing and takeoff at both sites.

During Bush's visit, a security cordon will be set up over a radius of some 300 meters around each landing site, said Palestinian police spokesman Adnan Damiri.