WASHINGTON (CNN) -- The U.S. military faces between 13,000 and 17,000 insurgents in Iraq, the large majority of them backers of ousted Iraqi leader Saddam Hussein and his Baath Party, a senior military official said Tuesday.

Those figures came to light the same day an apparent suicide bombing killed more than 20 people in central Baghdad as they waited in line to apply to be police officers, U.S. and Iraqi officials said.

The bulk of the insurgency is made up of 12,000 to 15,000 Arab Sunni followers of Saddam's party, the official told CNN. The Baath Party was overthrown by a U.S.-led invasion in March 2003.

Of those, the source said 5,000 to 7,000 are considered "committed" fighters, with the rest considered "fence-sitters," criminals or "facilitators" who contribute material support or sanctuary to the guerrillas.

The official, who is familiar with the region, said about 500 other fighters have come from other countries to battle the U.S.-led forces in Iraq, while another group of fewer than 1,000 are believed to be followers of Jordanian-born Islamic terrorist Abu Musab al-Zarqawi.

Members of Congress have been pressing senior officers for an assessment of the strength of the insurgency since Iraq's January 30 elections.

At a Senate Armed Services Committee hearing last week, Sen. John McCain criticized Gen. Richard Myers, chairman of the Joint Chiefs of Staff, for lacking a readily available estimate of the armed opposition.

"I don't know how you defeat an insurgency unless you have some handle on the number of people that you are facing," the Arizona Republican said.

Told by Myers that some numbers exist but are classified, McCain said, "I think the American people should know the extent of the enemy we are facing."

The numbers are considerably higher than the 5,000 fighters that Gen. John Abizaid, head of the U.S. Central Command, estimated in November 2003. The Pentagon cautioned, however, that trends are difficult to track.

The official who provided Tuesday's estimate said the U.S. military believes it killed between 10,000 and 15,000 guerillas in combat last year -- perhaps as many as 3,000 during the November push to retake the western Iraqi city of Falluja from insurgents.

But because others join the insurgency to replace those killed, Pentagon analysts have difficulty matching the current number against previous assessments.

In the wake of the elections, in which Iraqis turned out to vote for a transitional parliament, U.S. commanders expressed hope that Iraqis will rethink their commitment to the insurgency.

A long time ago it was a few hundred "dead-enders" followed by a couple thousand "dead-enders". We've probably killed a couple thousand insurgents... yet now there's 17k more? Spiffy.

Yeah, but what surprises me is that this is saying that a grand majority of them are former Saddam loyalists. I would have thought that a lot of them were foreign or recruited by Zarqawi, et al.

That doesn't surprise me at all. There are clearly foreigners fighting in Iraq, but just imagine 20,000 foreign fighters surrepticiously crossing the border into Iraq over the past year. That's a lot of people, even with Iraq's porous borders. It only makes sense that the vast majority of the fighters were locals. They may not Saddam loyalists necessarily, but they're definitely Iraqis.

--PunkDavid

I think I made that "or" a bit unclear. I was saying I thought they'd be either foreign OR Iraqis recruited by other groups. I'd say a minority of them are foreigners as well, but for some reason I'm guessing that border security isn't exactly perfect in Iraq right now.

That isn't specific enough as it technically describes both sides.

I wish the media would stop using the word "insurgents" and use the proper word: terrorists

I'm embarrassed for you.

I'm embarrassed for you.

--PunkDavid