Aiming for Saddam Hussein's seat of power, coalition planes targeted Republican Guard forces just south of Baghdad in perhaps the largest assault to date on Saddam Hussein's highly trained troops, as U.S. officials warned that Iraq could use chemical weapons if soldiers crossed a "red line" ringing the capital.

Troops pressed toward Baghdad but many were but many were halted by a sandstorm near the holy city of Karbala, 50 miles south of the capital.

Coalition farther south were facing deadly ambushes and discovering that many Iraqi fighters had discarded their uniforms for civilian clothes.

U.S. General Tommy Franks said Saddam's regime was weakened but still issuing orders to military units - which are not always complying. Franks said his forces have captured 3,000 prisoners.

Pentagon officials said U.S. helicopters had begun attacking Saddam's forces arrayed around the city.

"They [Republican Guard units] have been hit, they will continue to be hit... at points and places and times that make sense to us," said U.S. Army General Tommy Franks, commander of the coalition forces operating in Iraq.

Allied troops' entry into Baghdad itself, however, could be met with a chemical attack.

American television stations CNN, NBC and CBS reported Monday that U.S. officials said the Iraqi leadership has drawn "a red line" around the map of Baghdad and that once American troops cross it, Iraqi Republican Guards have been authorized to use chemical weapons, Reuters reported.

While a senior Pentagon source said he could not confirm the reports, NBC said its information was coming from intelligence officials who based it on intercepts of Iraqi communications.

"It's believed once U.S. ground troops cross the line drawn roughly between Karbala and Al Kut, the Republican Guards are under orders to attack with chemical weapons," NBC said.

CNN said the fact that the use of the weapons had apparently been authorized did not mean they would ultimately be used.

British Prime Minister Tony Blair, who will meet with U.S. President George W. Bush in Washington later this week, told the British parliament that the "vital goal" was to reach the Iraqi capital as swiftly as possible.

At about the same time as the Baghdad attack, heavy bombing was heard from near the key northern city of Mosul, indicating that coalition forces were hitting positions closer to the border with the Kurdish area.

Earlier Monday, Franks said that the fate of a two-member crew of a U.S. Apache attack helicopter that went down during fighting south of Baghdad was "uncertain" and that the two men were classified as "missing in action."

Militia fighters and irregular Iraqi forces held off British forces outside Basra on Monday, amid heavy fire from mortars and rocket-propelled grenades and dogged efforts to prevent coalition troops from taking Iraq's second largest city.

Near Basra, a British soldier was reported killed in action and two others were missing.

In the southern oil fields, once considered secure, shadowy Iraqi forces apparently ambushed a British unit by feigning surrender.

Warnings intensified of a humanitarian crisis as fighting in the south delayed entry of aid and water grew short in Basra. UN Secretary-General Kofi Annan called for urgent action to make sure there was enough water in Basra, a city of some two million people.

Two men missing in action Iraqi television showed on Monday night a video of two men it said were the crew of the downed helicopter. The pilots - whom the Pentagon identified as chief warrant officers David Williams, 30, of Florida, and Ronald Young Jr., 26, of Lithia Springs, Georgia - were seen sitting and both appeared to be in good health.

"The fate of the crew is uncertain right now," Franks told a news briefing at Central Command in Qatar. "We characterize that crew, two men, as missing in action."

He said the helicopter was one of between 30 and 40 attacking a particular area. He denied that it had been shot down by farmers, as Iraqi television maintained, but did not say what had forced it out of the air in Iraqi-held territory.

Iraqi television showed the identity papers of the two men and said their helicopter had been shot down by a farmer. The television did not show them saying anything, and they were not questioned on video.

A U.S. defense official confirmed earlier in the day that one U.S. Apache Longbow helicopter was downed in Iraq, but had no comment on claims by Iraq that it had shot down a second attack helicopter.

"I can confirm that we have one helicopter down," said the official, but he had no information on the fate of the two U.S. pilots aboard the helicopter, which was shown on Iraqi television near the city of Kerbala, 120km (70 miles) southwest of Baghdad.

Iraq claims to have shot down two Apache helicopters and to be holding two pilots prisoner.

Two helmets were shown at the scene but there was no apparent sign of the crew of the twin-engine army attack helicopter, which was previously deployed in the 1991 Gulf War, Bosnia, Kosovo and Afghanistan.

First British soldier confirmed killed in combat The British military said Monday that one of its soldiers had been killed in action near Zubayr, between the southern Iraqi city of Basra and the Kuwaiti border. This is the first British soldier to be killed in combat.

"We regret to announce that a British soldier was killed in action today near Zubayr in southern Iraq," said a British military official at Central Command in Qatar.

Earlier Monday, British defense officials said that two British soldiers were missing after vehicles they were traveling in were attacked in southern Iraq. It is unclear if the two incidents are linked.

The Ministry of Defense said in a statement the attack took place Sunday, but officials declined to give details of to which unit the soldiers belonged and would not elaborate on where the attack took place. "Every effort is being made to find and recover the soldiers," the ministry said.

A spokesman said the ministry was being deliberately vague about the suspected location of the soldiers for safety and operational reasons. No further details were immediately available.