A Reuters photographer and driver were killed in Baghdad Thursday.

Namir Noor-Eldeen, 22 years old, and 40-year-old driver Saeed Chmagh, a father of four, were working in eastern Baghdad reporting on clashes between militants and the U.S. military. The exact cause of their deaths is not known, but Iraqi police said either a mortar attack or U.S. air strike could be to blame.

Their deaths bring the number of Reuters journalists killed in Iraq to six, since 2003.

"Noor-Eldeen and Chmagh's outstanding contribution to reporting on the unfolding events in Iraq has been vital. They stand alongside other colleagues in Reuters who have died doing a job that they believe in,"said Tom Glocer, chief executive of Reuters, which will hold a one-minute silence Monday in tribute to the men.

According to Reporters Without Borders, 53 journalists have been killed worldwide so far this year.