Numaniyah, Iraq ? As U.S. troops move closer to Baghdad, they are crossing what commanders call a “red line” beyond which Iraqi President Saddam Hussein is most likely to use chemical or biological weapons.

Marines have been wary of chemical and biological weapons as they have moved north through central Iraq, wearing their protective suits around the clock. So far, there has no hint of a chemical attack, but Marine commanders say that as they cross the Tigris River and move north, while Iraq’s military position continues to erode, a chemical assault could be imminent.

“There is some red line in his psyche, probably not necessarily physical, but he will reach the point when he will use it,” said Lt. Col. Michael Belcher, commander of the 3rd Battalion, 7th Marines.

“It may be terrain, it may be operational, it may be political. All I can believe is that the closer we get to Baghdad, the closer we are to reaching that line.”

Capt. Lyle McDaniel, the battalion’s intelligence officer, said if Saddam planned to use chemical or biological or chemical weapons, he must do it now because he was running out of time.

“I didn’t think he would use it initially, in the beginning he has an international political edge,” said McDaniel, of Beaufort, S.C. “If he had used them then, he would have lost that.”

“But the closer we put him in a dilemma where he’s in a lose-lose situation, he’s more likely to use them.”

McDaniel and other military officials in Iraq think crossing the Tigris River could be the decisive line.

“After crossing the Tigris, there is an open area into Baghdad,” McDaniel said. “Then, the only thing between us and Baghdad is the Republican Guard. At that point, we’re putting him in the dilemma that it’s ‘do whatever you can or lose.”‘

British soldiers wear gas masks during a chemical alert in a location near Basra, in southern Iraq. The alert was issued Tuesday. Farther north, U.S. troops are keeping their protective suits ready in the event of a chemical or biological attack as they near Baghdad.

Marine commanders will place their men on highest state of chemical and biological readiness for any further moves north. Marines are prepared to fight for 24 hours in gas masks and chemical suits after a gas attack before decontaminating. Belcher said Marines already had enough replacement suits available to refit the troops and continue their march toward Baghdad.

Marines have spent numerous hours in classes on how to respond to chemical attack.

Since staging in Kuwait in January, all Marines have been required to have their gas masks with them 24 hours a day. They must also have instant access to rubber boots and gloves that they have been issued.

When Marines initially crossed into Iraq on March 20, there were a number of false alarms. In the past 10 days, there has been little mention of chemical assault.

Republican Guard units are said to possess chemical weapons, supposedly loaded on artillery and rockets that have a range of about 12 miles or less. Coalition troops, however, have not found chemical stockpiles so far, and Saddam has repeatedly denied having any — although Iraq did acknowledge it had developed such weapons before the 1991 Gulf War and used them in the 1980s.

“We keep seeing chatter in the intelligence channels about the possibility” of a chemical strike, Defense Secretary Donald H. Rumsfeld told reporters. He said the war commander, Gen. Tommy Franks, “has thoughts about a way to dissuade and deter further use of chemical weapons, but we’ll leave that to the future.”

Among the outlawed chemical weapons Iraq is said to have are deadly nerve gases sarin, cyclosarin and VX, and a mustard agent like that used in World War I.