NEAR FALLUJAH, Iraq -- U.S. soldiers and Marines launched a large attack Thursday into the southern half of Fallujah, where insurgents still holding, opening a second phase in the offensive, U.S. officials said.

Slideshow: Forces Make Progress In Fallujah Slideshow: Images From Battle Of Fallujah

The news follows reports that more than 200 U.S. soldiers have been seriously hurt in the offensive and flown to Landstuhl Medical Center in Germany.

The Center is usually used to treat severely wounded troops, but there are no official casualty figures yet from the military about the battle for Fallujah.

Two planes carrying about 90 wounded Marines and soldiers were due in from Iraq today. Those are troops too seriously injured to be treated at U.S military hospitals in Iraq.

A spokeswoman says doctors there are "very busy."

The large number of wounded sent to Germany suggests that fighting may be more intense - at least in some areas - than the military had initially indicated.

The new assault began around sundown and followed three days of fighting in the northern districts.

U.S. troops launched the first major assault against Fallujah late Monday, pushing into northern districts until they reached the main east-west highway that bisects the city.

Fighting has been underway since then to clear pockets of resistance from northern areas, where insurgent positions were believed the strongest.

Commanders say their troops have cordoned off the entire city to prevent fighters from escaping.