The U.S. government on Friday proposed prohibiting commercial truck and bus drivers from using cellphones while behind the wheel.

The Transportation Department rule would affect approximately 4 million drivers, who are already banned by the government from texting while working.

The proposal is the latest move in Transportation Secretary Ray LaHood's stepped-up campaign against distracted driving in which he has questioned cellphone use in passenger cars and hands free communications technology.

Every time a commercial truck or bus driver takes his or her eyes off the road to use a cellphone, even for a few seconds, the driver places everyone around them at risk, LaHood said in a statement.

Nearly 5,000 people were killed and another 500,000 were hurt in crashes of all vehicles involving a distracted driver in 2009, government safety figures show.

Inattention was a factor in 9 percent of large truck crashes, which fell overall in 2009 from the previous year. Most truck crashes involve collisions with other vehicles.

Fatal bus crashes jumped from 20 to 38 in 2009.

Between 6 percent and 13 percent of bus crashes were linked to inattention, according to a National Transportation Safety Board investigation of those types of accidents between 1998-2008.

Many big transport companies, like UPS Inc and Wal-Mart Stores Inc, already prohibit drivers from using a cellphone while operating their trucks.

The cellphone proposal is subject to a 60-day comment period before being finalized.

(Reporting by John Crawley; editing by Philip Barbara)