"All of them were killed by the American army," Reuters chief David Schlesinger told reporters on Thursday on the sidelines of a media conference in the southern Portuguese resort of Vilamoura, national news agency Lusa reported.
"There is no understanding on the part of the US military regarding the exercise of journalism," he added.
"We can't run the risk that journalists will become targets (in Iraq). We must learn the lessons from these tragic cases."
Two Reuters photographers and a cameraman are among the more than 60 war-related deaths of media workers recorded in Iraq. The most recent death occurred in the Iraqi city of Ramadi on 1 November.
The US military says a cameraman killed there while on assignment for Reuters died in a gunbattle between Marines and fighters, but the Iraqi man's colleagues and family have said they believe he was shot by a US sniper.
Another Reuters cameraman, a Ukrainian citizen, was killed in April 2003 when a US Army tank fired on the Palestine Hotel in Baghdad.
A cameraman from a Spanish
network was killed in a US strike
A cameraman from Spain's Telecinco television network was also killed in the strike, which injured three other reporters.
In October 2003 a Palestinian cameraman for Reuters was killed near Abu Ghraib prison during a shootout.
The US military has denied direct responsibility for those deaths as well. Pentagon spokesman Bryan Whitman told the media conference via satellite from Washington that those incidents were inevitable in a war.
"Media coverage in places of conflict is always dangerous," Lusa quoted him as saying.
He put the blame for the two deaths at the Palestine Hotel on Iraqi troops resisting the US invasion, whom he accused of using civilian structures for military purposes, leading to confusion about what is a legitimate target.
Journalists at the Palestine Hotel, including many working for US-based organisations, had informed US military authorities that they were using the hotel as a base.