Advancing on Saddam Hussein's hub of power, coalition planes and helicopters pummeled Iraqi Republican Guard forces just south of Baghdad into Tuesday as ground forces hunkered down 50 miles from the capital city in the midst of blinding sandstorms.

U.S. forces are using air power, helicopters and artillery to "beat down those Republican Guard positions" before ground troops make their move on Baghdad, the chairman of the Joint Chiefs of Staff said Tuesday.

"I wouldn't characterize any of that as a pause; it's all part of a plan," Air Force Gen. Richard Myers said on ABC's "Good Morning America."

"We've never said it's going to be quick, we've never said it's going to be easy, you've never heard those words come out of officials here at the Pentagon," he said. "War is tough and it's going to be a tough fight. But it's a worthy fight."

Allied ground troops also crossed over the Euphrates River early Tuesday, advancing north past Nasiriyah after facing fierce resistance there. But the intense sandstorms in the region may slow the troops down as they move toward the capital.

U.S. officials confirmed that there is growing worry in the Pentagon that Iraqi Republican Guard units will start using chemical weapons as coalition land forces approach Baghdad.

The officials cited multiple intelligence reports that Iraqi units may have been ordered to unleash chemical weapons, but cautioned that reports of specific geographic tripwires, or "red lines" drawn up by Iraqi leadership, are premature.

The Iraqi Republican Guard controls the bulk of Iraq's chemical weaponry, most of which can be fired from artillery guns or short-range rocket launchers, according to U.S. officials.

Secretary of State Colin Powell told Fox News' Brit Hume Monday that it's no surprise Saddam would stoop so low.

"I have no doubt that he would do such a thing," Powell said. "We are concerned about it -- we will follow this matter carefully."

But coalition leaders remained confident that Saddam's days are over.

"There will be resistance all the way to the end of this campaign," British Prime Minister Tony Blair said Tuesday. "The resistance will be broken down."

To the people of Iraq, he said: "This time, we will not let you down. Saddam and his regime will be removed -- Iraq will have a better future."

As explosions grew in frequency, people in Baghdad were carving more and bigger defensive trenches around the city, including in the courtyard of the Iraq museum, home to priceless antiquities, some dating to 7,000 B.C. But many more stores were open for business and people milled about.

An intense coalition air strike Monday marked the first known engagement between forces in central Iraq. Many coalition aircraft were hit by Iraqi groundfire. One went down behind enemy lines and the Pentagon said the two-person crew had been taken prisoner.

"These things are never easy," conceded Blair, after his country suffered its first combat casualty of the war. "There will be some difficult times ahead but [the war] is going to plan despite the tragedies."

Later on Tuesday, British officials confirmed the nation's second combat casualty: a soldier from the First Battalion Black Watch, whose members are mostly from Scotland, was killed overnight near the town of Az Zubayr, close to Basra.

Iraqi trade minister Mohammad Mehdi Saleh on Tuesday said Iraq was a "rich country" that didn't require humanitarian aid, while simultaneously accusing the United States and Britain of "preventing the arrival of food and medicine" to the nation.

The U.N. Oil-for-Food Program allows Iraq, which has been under heavy economic sanction since the first Gulf War, to sell oil as long as the profits are used to feed and care for Iraqis. Saleh claimed that the United Nations had illegally dismantled the relief program under U.S.-British pressure and that supplies designated for Iraq were being turned away at the borders.

But a U.N. spokeswoman said supplies hadn't arrived because ground fighting was keeping relief workers away.

"It's a very dangerous and volatile situation right now," she told the BBC. "We have enough food to eventually assist most of the Iraqi population ... We could be taking food by road through Kuwait.

Blair on Tuesday said it's Saddam's fault humanitarian aid hasn't yet reached the Iraqis. Coalition forces are still trying to de-mine waterways leading to Iraq.

There are "huge stockpiles" of aid in Kuwait, waiting to be deployed to the port of Umm Qasr and other places, Blair said.

"All that's holding us back is the threat of Iraqi mines and it will take us some time before we can make it safe."

A military barracks in the northern part of the country was bombed Monday, and Baghdad fell under renewed air attack by day and by night.

Iraqis set up mortar positions south of the city and piled sandbags around government buildings and other strategic locations.

U.S. Apache helicopters attacked Saddam's Republican Guard forces arrayed around Baghdad while an official said a "large portion" of the day's bombing runs -- there were more than 1,500 sorties over Iraq -- were dedicated to hitting the same units.

The Apaches encountered heavy groundfire during their assault on the Medina armored division, but the helicopters managed to kill about 10 Iraqi tanks before cutting off their attack.

The Pentagon says 80 percent of its bombs and missiles are precision-guided, but Iraq claimed that 58 civilians had been killed Sunday and 469 injured.

Heavy fighting continued Monday in An Nasiriyah.

It seems only a matter of time until coalition ground forces are engaged in direct firefight with Republican Guard forces.

The Army's 3rd Infantry Division was within hours of the capital, battling sandstorms more than Iraqi fire.

Some Iraqis waved or gave a thumbs-up as the convoy passed on its dash through southern Iraq, while others stood stoically.

The advance of thousands of vehicles was aided by heavy air protection that wiped out a column of Iraqi armor and sent some outer defenses withdrawing toward the capital.

President Bush on Monday ask Congress for nearly $75 billion, $62.6 billion of which would be in direct war costs for 30 days of combat.

The request was also expected to include up to $3 billion to guard against terrorist threats, as well as aid to Israel, Afghanistan and other U.S. allies; a down payment on humanitarian aid for Iraq and for rebuilding the country and money to increase security for American diplomats.

Bush is scheduled to confer in Washington Wednesday with Blair. National security adviser Condoleezza Rice visits U.N. Secretary-General Kofi Annan on Tuesday to discuss humanitarian issues.

Polls show growing support for the military campaign.

In the world's first war with live broadcasts from the battlefield, news and images of American and British setbacks competed with pictures of military successes.

U.S. Central Command chief Tommy Franks confirmed there was a two-man crew missing. Two Apache pilots were shows as prisoners on Iraqi TV. The U.S. Army found the downed Apache Monday night and destroyed it.

In other news, NBC News reported that Al Qaeda has sent an Arabic message to Muslims in Iraq that is basically a military playbook on how to defeat Americans.

The message is addressed to "our brothers in Iraq" and was written by Saif Al-Adel, the security chief for Usama bin Laden. He's believed to be the third highest Al Qaeda official alive, and on the run. Al-Adel lays out lessons learned fighting Americans in Afghanistan, and claims "victory over the U.S. [in Iraq] is very possible ... easy beyond the imagination" and depends on "depleting, exhausting and terrorizing the enemy."

Fox News' Kelly Wright, Greg Kelly and The Associated Press contributed to this report.