NATANZ, Iran — Iran announced a dramatic expansion of uranium enrichment Monday, saying it has begun operating 3,000 centrifuges — nearly 10 times the previously known number — in defiance of U.N. demands it halt its nuclear program or face increased sanctions.

U.S. experts say 3,000 centrifuges are in theory enough to produce a nuclear weapon, perhaps within a year. But they doubted Iran really had so many up and running, a difficult technical feat given the country's spotty success with a much smaller number.

Instead, the announcement may aim to increase support at home amid growing criticism of hard-line President Mahmoud Ahmadinejad and to boost Iran's hand with the West by presenting its program as established, said Michael Levi, a nonproliferation expert at the Washington-based Council on Foreign Relations.

"From a political perspective, it's more important to have (3,000 centrifuges) in place than to have them run properly," Levi told The Associated Press. "We have an unfortunate habit to take Iran at its word when they make scary announcements."

The White House and Europe criticized the latest announcement.

4 GUARD UNITS TAPPED FOR POSSIBLE IRAQ DUTY

WASHINGTON — Some 13,000 National Guard troops are receiving notice to prepare for possible deployment to Iraq, which would be the second tour for several thousand of them.

The orders had been anticipated, but the specific units were not announced until Monday. They are the Army National Guard's 39th Infantry Brigade Combat Team, based in Little Rock, Ark.; 45th Infantry Brigade Combat Team, Oklahoma City; the 76th Infantry Brigade Combat Team, Indianapolis, and the 37th Infantry Brigade Combat Team, Columbus, Ohio.

The units would serve as replacement forces in the regular troop rotation for the war, and would not be connected to the recent military buildup for security operations in Baghdad, the Pentagon said.

One unit would deploy in December and the others in 2008, the Army said.

"They are receiving alert orders now in order to provide them the maximum time to complete their preparations," the Defense Department said in a separate statement. "It also provides a greater measure of predictability for family members and flexibility for employers to plan for military service of their employees."

SHIITES MARCH TO DEMAND U.S. LEAVES IRAQ

BAGHDAD — Tens of thousands of Shiites — a sea of women in black abayas and men waving Iraqi flags — rallied Monday to demand that U.S. forces leave their country. Some ripped apart American flags and tromped across a Stars and Stripes rug.

The protesters marched about three miles between the holy cities of Kufa and Najaf to mark the fourth anniversary of the fall of Baghdad. In the capital, streets were silent and empty under a hastily imposed 24-hour driving ban.

Radical Shiite cleric Muqtada al-Sadr ordered up the march as a show of strength not only to Washington but to Iraq's establishment Shiite ayatollahs as well.

Al-Sadr, who disappointed followers hoping he might appear after months in seclusion, has pounded his anti-American theme in a series of written statements. The most recent came on Sunday, when he called on his Mahdi Army militia to redouble efforts to expel American forces and for the police and army to join the struggle against "your archenemy."

The fiery cleric owes much of his large following to the high esteem in which Shiites hold his father, Ayatollah Mohammed Sadiq al-Sadr, who was assassinated in 1999 by suspected agents of Saddam Hussein. Al-Sadr dropped from view before the start of the latest Baghdad security operation on Feb. 14. U.S. officials say he is holed up in Iran. His followers insist he's returned to Najaf.

BUSH TOUTS IMMIGRATION PLAN IN BORDER STATES

YUMA, Ariz. — President Bush visited the U.S.-Mexico border Monday to tout a guest worker program for immigrants, pursuing a key domestic policy goal despite chilly relations with Congress.

The trip, a bookend to the visit that Bush made to the same southwest desert city last May, comes as tension rises over a new immigration proposal tied to the White House. Bush's team is privately working hard to rally votes for what Bush calls comprehensive reform — a mix of get-tough security with promises of fair treatment for undocumented residents.

Upon arriving in Yuma, Bush met Homeland Security Secretary Michael Chertoff. The two took a quick look at the "Predator," an unmanned plane that border officials use to monitor the region.

Bush pointed to two new layers of fencing that have been erected at the border since he visited the same spot a year ago.

"It's amazing the progress that's been made," Bush told border officials. "I was most impressed by your strategy, but more impressed by the fact that it's now being implemented."

RADIO STAR IMUS FIGHTING FOR JOB AFTER INSULT

NEW YORK — After a career of cranky insults, radio star Don Imus was fighting for his job Monday following one joke that by his own admission went "way too far."

Two of the nation's biggest media companies — CBS Corp. and NBC Universal — will ultimately decide the fate of Imus' daily program after he referred last week to members of the Rutgers women's basketball team as "nappy-headed hos."

Imus continued to apologize Monday, both on his show and on a syndicated radio program hosted by the Rev. Al Sharpton, who is among several black leaders demanding his ouster.

Imus could be in real danger if the outcry causes advertisers to shy away from him, said Tom Taylor, editor of the trade publication Inside Radio.

"Everyone is on tenterhooks waiting to see whether it grows and whether the protest gets picked up more broadly," Taylor said.

DEFORESTATION CAN HELP, HURT CLIMATE

WASHINGTON — The effect of deforestation on climate depends on three things — location, location and location.

Environmentalists concerned about global warming have long encouraged preservation of forests because they absorb carbon dioxide, the most abundant greenhouse gas in the atmosphere.

But the issue, like most things, may be more complicated than it first appears.

New research in this week's online edition of Proceedings of the National Academy of Sciences, confirms the effectiveness of tropical forests at reducing warming by absorbing carbon. But it suggests that in snowy latitudes, forests may actually increase local warming by absorbing solar energy that would otherwise be reflected back out into space.

That doesn't mean forests in cold areas should be chopped down, stressed Ken Caldeira of the Carnegie Institution department of global ecology, located at Stanford University in Palo Alto, Calif.

STEEP FEES FOR DUMPING INTERNET SERVICE PROVIDER

WASHINGTON — Breaking up with your Internet service provider isn't hard to do — but it may cost you.

Customers who subscribe to a high-speed Internet plan may pay $150 or more if they terminate their service before their contract has expired, according to a new survey from Consumers Union, the nonprofit publisher of Consumer Reports magazine.

The practice is well known among cell phone providers — early termination fees in that industry run from $150 to $240 per line, according to the group.

The goals of the fees are largely the same — to cut down on "churn," the process of customers dumping one service provider to pursue greener pastures with another.