"Digging it out, ferreting it out is going to take some time," said the official, who asked not to be identified. "It's going to be difficult, it's not going to be a cakewalk."

The United States and Britain used accusations of a hidden weapons of mass destruction program as the primary justification for invading Iraq. But so far no chemical, biological or nuclear weapons have been officially reported.

Hans Blix, the chief UN weapons inspector, has suggested sending back UN inspectors. Blix told the BBC on Thursday they would give the world a "credible report on the absence or the eradication of the program of weapons of mass destruction."

The White House, however, has said it was not yet time to discuss the return of UN inspectors, and the large-scale US effort signalled that the United States wants to conduct its own search.

"We both want the same thing," said the defense official, referring to Blix's call for a return of UN inspectors. "So I can forsee some collaboration, but that's not something we've talked about at this time."

The new Iraq Survey Group will expand efforts currently underway by a couple of hundred US military personnel with 75th Exploitiation Task Force, a specialized group assigned the task of finding and securing potential weapons of mass destruction sites.

They have visited only about 50 venues on a list of hundreds of sites related to Iraq's weapons of mass destruction program, the defense official said.

It was not immediately clear when the larger group of experts, who are mainly civilians but include some military personnel, will arrive in Iraq.

The defense official said the military has held off bringing them in until conditions inside the country are stable enough to allow them to work safely.

Thirty to 40 former UN arms inspectors have been asked to join the team, which also includes former and current US government experts and officials.

SPACE.WIRE