Defense Secretary Donald Rumsfeld warned Syria on Friday to stop supplying Iraq with weapons to use against coalition forces.
Fox News reported Thursday that Defense officials said they were seeing "military equipment" - "night-vision goggles and other things" - moving from Syria to Iraq, but would not elaborate on the size of the shipments or if they were affecting the military situation.
Rumsfeld affirmed Friday that Syria was in fact engaging in such activities, and sent out a stern warning.
"These deliveries pose a direct threat to the lives of coalition forces," Rumsfeld said at a Pentagon briefing. "We consider such trafficking as hostile acts, and will hold the Syrian government responsible for the incidents."
When asked if the shipments from Syria were "state-sponsored," Rumsfeld said he wouldn't answer because "it's an intelligence issue."
"They control their border," he added. "We're hoping that kind of thing doesn't happen."
Syrian Foreign Ministry spokeswoman Bouthaine Shaban rejected Rumsfeld's statement as "unfounded and irresponsible."
"He only brings problems for his country and humanity at large," she told Britain's Channel 4 television in a telephone interview from Damascus. "It is an absolutely unfounded, irresponsible statement, just like his statements that brought his country and the allied countries into a terrible war, unnecessary war on Iraq."
Syria later dismissed the U.S. accusations and said they were an attempt to divert attention from the number of Iraqis dying in the war.
Syria has effectively opened its borders for volunteers trying to get into Iraq to fight for Saddam Hussein's regime, as well as refugees trying to escape, U.S. government officials confirmed to Fox News Friday.
Some citizens in Saudi Arabia and Lebanon also reportedly want to take up arms for Saddam.
"There's no question but that to the extent that military supplies or equipment or people are moving across the borders between Iraq and Syria, it vastly complicates our situation," Rumsfeld said.
Asked if the United States was threatening military action against Syria, Rumsfeld said: "I'm saying exactly what I'm saying. It was carefully phrased."
On the flip side, Rumsfeld said Iraqis opposed to Saddam's regime were streaming into Iraq from Iran, where they had been in exile.
By contrast, officials say the Turkish, Iranian, Jordanian and Saudi Arabian borders were closed and more carefully watched. Leaders of the Palestinian Islamic fundamentalist terrorist group Hamas have distributed pamphlets in the West Bank demanding that Jordan and Saudi Arabia open their borders.
Officials said the U.S. government's concerns have been communicated to the Syrian government, though exactly how was unclear.
The United States has said Russia has also provided Iraq with night-vision goggles, as well as electronic jamming equipment that could alter the path of U.S. bombs and aircraft.
Sources say Hezbollah fighters from Lebanon were also en route to Iraq.
U.S. control of western Iraq was expected to neutralize the threat, if any, these volunteers might present to coalition efforts.
"We've seen this kind of example before," Brig. Gen. Vince Brooks said at U.S. Central Command Friday, referring to Afghanistan. "Some are very interested in deliberation that's ongoing ... we have not had any encounters and it has not posed a problem to date."
Syria staunchly opposed any sort of military action in Iraq during the last few months of diplomatic wrangling at the U.N.
Syrian President Bashar Assad has described the military action against Iraq as "clear occupation and a flagrant aggression against a United Nations member state."
To figure out exactly who has been supplying Iraq with military equipment, U.S. officials were investigating arms dealers, transactions and bank accounts in several countries, including Yemen, Syria, Russia, Turkey, Jordan, Romania, Yugoslavia, Poland, Bulgaria, the Czech Republic, Slovakia, Ukraine and Belarus.
On Thursday, Mufti of Syria Sheikh Ahmad Kaftaro, the country's top official Muslim religious authority and a man generally regarded as a moderate, urged homicide bombings against coalition forces in Iraq, reported Agence France Presse.
"I call on Muslims everywhere to use all means possible to thwart the aggression, including martyr operations against the belligerent American, British and Zionist invaders," Kaftaro said in a statement. "Resistance to the belligerent invaders is an obligation for all Muslims, starting with [those in] Iraq."
U.S. officials were particularly concerned about the increasingly anti-American and anti-British tone in the Syrian government-controlled press.
Officials from the Syrian Foreign Ministry are quoted in newspapers this week as calling the United States the "the empire of murder and destruction."
Syria has long regarded homicide attacks by Palestinians against Israel as legitimate "resistance to occupation."
Kaftaro, the leader of the Naqshbandi Sufi sect, called on Muslims everywhere to boycott U.S. and British products, as well as goods from countries supporting the coalition war effort.
He also urged "free citizens" all over the world to demonstrate, strike and disrupt airports and ports and other facilities that support the war effort in Iraq.
Separately, Syrian President Bashar al-Assad, in an interview published Thursday in the Lebanese daily As-Safir, predicted the United States and Britain would never be able to bring Iraq under their full control and would face "popular Arab resistance."
Even Arab newspapers now appear to be praising Saddam's ability to fight back.
"Six days have passed and not one Iraqi city has fallen to the invaders," wrote Mahmoud Abdel Moneim, a columnist for the al-Ahram daily newspaper in Cairo, Egypt. "Saddam Hussein hasn't fled, and he hasn't given in. The whole world mocks the allies and their weapons and their numbers, which are incapable of achieving any victory over Iraqi forces who are supported by no one."
A commentary in the Tishrin daily in Syria on Thursday indicated that - in the words of a writer for the Syrian Web site - "there is a complete conviction in the world that what is going in Iraq is more than an ordinary aggression and the American strategy in targeting the Iraqi people, land and resources."
The newspaper stressed that the "U.S. war machine doesn't hesitate to target the Iraqi civilians with round-the-clock missiles and bombs in the most brutal raids ever in history, particularly on Baghdad."
The Al-Thawra daily opined that the Iraqi people had proven their strength, and the al-Baath daily said the popular resistance of the Iraqi people would be an important factor in the Arab world.
Fox News' Bret Baier, Major Garrett, Carl Cameron, Liza Porteus and the Associated Press contributed to this report.