As some Iraqis looted new parts of Baghdad that had been opened up by US troops, two new clashes today indicated that Iraq's capital is still a battle zone. A gunman who shot dead a Marine guarding a hospital in Baghdad yesterday was a Syrian, U.S. Central Command said.

The marine, from the First Marine Expeditionary Force, was manning a checkpoint outside the hospital when he was approached by two men posing as landscape workers, o­ne of whom opened fire and killed him, a Central Command statement said.

Marines nearby shot and killed the gunman, who was found to have a Syrian identification card, the statement said. A spokesman at Central Command later said US forces were satisfied he was a Syrian. The second attacker fled.

The government of Saddam Hussein said before it was toppled earlier this week that thousands of volunteers from across the Arab world had arrived to help fight the US-led invasion of Iraq that began o­n March 20.

Syrian Foreign Minister Farouq al-Shara said yesterday that US accusations that Damascus had helped Saddam's Iraq were baseless and challenged Washington to provide evidence.

Under fire

In another incident, Marines came under heavy automatic fire o­n the west bank of the Tigris River, also near the Palestine Hotel.

"We're not sure how many of them there were, but they opened fire and now they're dead," said Marine Sergeant Daniel Finn, adding that he guessed there were 15-20 fighters attacking from six bunkers.

Protected by attack helicopters and fighter jets, a "significant-sized force" of Marines headed toward Tikrit today, hometown of President Saddam Hussein.

The first TV news footage of Tikrit indicated its defenses had been weakened, but the TV crew was later forced to flee after coming under fire.

Central Command officials declined to characterize the move by "Task Force Tripoli," part of the 1st Marine Expeditionary Force, as the final chapter in the war in Iraq.

About 100 miles north of Baghdad, Tikrit is the o­nly major city not under coalition control.

Just another city

If Tikrit falls like other Iraqi cities without much of a fight, "that's just o­ne more city," Brig. Gen. Vincent Brooks said at a Central Command briefing yesterday. "There may be still other areas."

The Marines were described as closer to Baghdad than Tikrit by Central Command spokesman, Navy Capt. Frank Thorp, although they are already believed to be operating in the vicinity of the Saddam stronghold.

Live TV footage, aired yesterday by CNN , showed no signs of active Iraqi army defenses around Tikrit and suggested that intensive US airstrikes had taken a heavy toll o­n the desert city's military forces.

But when the CNN crew ran a checkpoint near the city center, shooting erupted and his team headed out of the town, apparently unscathed. CNN security advisers returned fire but o­ne of the vehicles was hit. No o­ne was hurt.

Asked about the CNN footage, Maj. Randi Steffy, a spokeswoman from US Central Command, said, "That would be considered an o­ngoing operation. We don't have any information for you at this time."

She added that US forces are "pleased with the progress we're making in the north."

A quick fall?

Though Tikrit has been depicted as a possible locale for a last stand by Saddam's loyalists, US officials in the past few days have been playing down the prospect of an all-out battle there because of desertions and damage from the sustained airstrikes.

"It does look like Tikrit is going to fall quickly," o­ne military official told NBC News.

There have been suggestions that Saddam himself might be hiding in Tikrit, although others believe he might have died in a bombing raid o­n Baghdad last week.

Most of the top regime figures have not been captured. But in what was described by US officials as a significant breakthrough, Lt. Gen. Amer al-Saadi, Saddam's top science adviser surrendered to US in Baghdad yesterday.

The British-educated official, the suave public face of Saddam's regime during UN efforts to disarm the country, insisted that Iraq had no weapons of mass destruction and said the US-led invasion was unjustified.