The U.S. military broadened its offensive in western Iraq on Monday, launching a major attack on insurgent positions in the town of Ubaydi near the Syrian border and killing about 50 insurgents in precision airstrikes and house-to-house street fighting, according to news reports and the U.S. military.

U.S. and Iraqi troops reportedly faced stiff resistance from machine-gun and small-arms fire and rocket-propelled grenades.

U.S. airstrikes hit five targets, the U.S. military reported. "The insurgents were engaging Coalition Forces with small arms fire at the time of the strikes," a U.S. military statement said.

A statement attributed to the insurgent group al Qaeda in Iraq was posted on mosques in Ramadi, about 140 miles away, saying 19 of its fighters had been killed. The group is led by Jordanian-born Abu Musab Zarqawi.

"This is a fight all the way through the city," said Col. Stephen Davis, commander of Marine Regimental Combat Team 2, describing the fighting, according to CNN, which had a producer embedded with U.S. troops. Davis said that his forces were encountering "significant resistance" and that they had found three buildings wired with explosives and numerous roadside bombs and car bombs. U.S. officials said about two dozen insurgents had been captured.

"Insurgent fighters have been battling with Iraqi and Coalition Forces since the operation began at dawn," a military statement said. "A suspected car bomb placed in the advance of Iraqi Forces was engaged with a round from an M1A1 tank. The blast from the tank initiated a secondary explosion powerful enough to throw the car onto the roof of a nearby building."

The fighting opened a new phase in Operation Steel Curtain, an offensive that began Nov. 5 in the border town of Husaybah, about 200 miles west of Baghdad. The offensive involves about 2,500 U.S. troops and 1,000 Iraqi army soldiers and is intended to rid the border area of havens for foreign insurgents, particularly members of al Qaeda in Iraq.

The U.S. military also is trying to smash a smuggling network used by al Qaeda to bring foreign fighters, money and weapons from Syria into the Iraqi heartland via the Euphrates River valley, which runs from the border almost to Baghdad, U.S. officials said.

At least two Marines have been killed in the operation and nine have been wounded, according to official reports.

In May, the U.S. military conducted a similar offensive, called Operation Matador, to clean insurgents out of Ubaydi, which is on the banks of the Euphrates about 15 miles from Syria. At least nine Marines were killed and 40 wounded in the operation, but insurgents apparently returned to the town afterward.

U.S. officials highlighted increased participation by Iraqi forces in the new offensive.

"Operation Steel Curtain differs from Matador in the respect that a permanent presence of Iraqi and U.S. forces will be established in the city," the military said in a statement Monday.

The U.S. military has not released comprehensive information about civilian casualties from the 10-day-old Steel Curtain offensive. U.S. military officials challenged a report by a Red Crescent Society official, quoted in The Washington Post last week, that 29 civilians were killed in the first three days of the operation. On Monday, CNN journalists embedded with U.S. troops filmed people picking through the rubble of a house where neighbors said 17 people had been killed in an airstrike, apparently several days ago.

In other violence Monday, a roadside bomb killed two South African security contractors in downtown Baghdad, a U.S. Embassy spokeswoman said. The contractors, who worked for Reston-based DynCorp International, a State Department security contractor, were killed when the bomb exploded near an Iraqi police checkpoint close to the fortified Green Zone, which houses the Iraqi government and U.S. diplomatic personnel. The spokeswoman said three people were injured in the blast.

Three other people were killed by a roadside bomb in eastern Baghdad, police said. In the insurgent stronghold of Ramadi, 60 miles west of the capital, three people were killed and 16 wounded when a roadside bomb exploded near a bus. Hamza Dulaimi, a manager at Ramadi Hospital, said the target of the bombing was apparently a U.S. military convoy traveling behind the bus, which he said was carrying students and employees of Anbar University.

An F-14D Tomcat taking off Sunday from the USS Theodore Roosevelt in the Persian Gulf. Airstrikes on Monday hit five targets, the U.S. military reported.