Amid a public debate over seat belts in school buses, the government is ordering a major review of school bus safety.

Federal regulators, in a report to Congress Friday, find that large school buses are one of the safest forms of transportation.But mindful that buses ferry "the nation's most precious cargo - children," the National Highway Traffic Safety Administration is conducting the first comprehensive review of school bus safety since the 1970s.

The $1 million, two-year review will examine all school bus crash data and agency crash testing and consider all possible safety upgrades, including padding the sides and roofs of buses or adding padded arm rests.

As the review begins, safety officials are writing guidelines recommending that new buses transporting children 4 or younger have lap belts or anchors to secure children or booster seats.

Agency officials said they probably would recommend changing only a few rows of seats because the seats would have to be spaced wider to accommodate child seats.

The recommendations would affect about 3 percent of the 23.5 million children who ride 440,000 school buses to and from class each day.

NHTSA contends school buses are among the safest vehicles on the road, noting that large buses are more massive than most vehicles they collide with and better able to absorb crash energy. The high-backed bus seats are padded and spaced about 24 inches apart to cushion students in a crash.

Only 11 children under age 19 are killed in school bus crashes each year, the report found. More children are killed each year - about 25 - trying to get off or on the bus, many of them hit either by a passing car or the bus.

Several groups have strongly advocated installing seat belts in buses, arguing that padded seats do not help when a large truck slams into the side of a school bus or the bus rolls over.

Government officials, though, worry that lap seat belts could cause children to double over, hitting their heads in accidents, while shoulder belts might be impractical to install.