"This behavior is completely unacceptable," Mr. Bush said in a joint news conference on his Texas ranch with Italian Premier Silvio Berlusconi. "States that continue to harbor terrorists will be held completely accountable."
The comment came after a lull in tough rhetoric against Syria and Iran. During the war and early occupation of Iraq, the Bush administration suspected Syria of harboring fugitive Iraqis and weapons, and charged that Iran was trying to influence events in occupied Iraq.
The U.S. moved some troops to the Iraq-Iran border to stop alleged incursions by Iranian agents, and briefly agreed to a cease-fire with an anti-Tehran terrorist group based in Iraq. The U.S. ultimately disarmed that group.
U.S. forces may have entered Syrian territory during a June 18 raid on a convoy of Iraqi trucks. Five Syrian soldiers who were captured in that raid were returned to Syria within days.
Last week, The New York Times reported that the CIA had blocked an administration official, State Department counter-proliferation head John Bolton, from telling Congress that Syria's biological and chemical weapons programs were a danger to the region. The intelligence agency disagreed with the assessment.
Syria and Iran have been on the U.S. list of state sponsors of terrorism for years.
Syria hasn't been directly linked to a terror act since 1986, but is listed because it allegedly provides " political and limited material support" to groups like the Popular Front for the Liberation of Palestine, the Palestine Islamic Jihad and Hamas.
Iran — a member of the "axis of evil" named by Mr. Bush in his 2002 State of the Union speech — is accused of providing "funding, safehaven, training, and weapons" to groups like Hamas, Islamic Jihad and Hezbollah.
Earlier, Mr. Bush and Berlusconi greeted each other like the oldest of friends when the Italian leader showed up for a two-day visit that followed his support for the Iraq war.
During their joint press appearance, Mr. Bush indicated that he had not yet made up his mind on the size of a U.S. force that might be sent to help peacekeepers from West African nations already in Liberia.
Bush reiterated his view that China, South Korea and other U.S. allies need to pressure North Korea to drop its nuclear ambitions and return to the negotiating table.
On Iraq, Bush urged allies to act under U.N. authority to help rebuild Iraq. "The more people involved in Iraq the better off we will be," he said. "A free Iraq is a crucial part of winning the war on terror."
For his part, Berlusconi talked of the importance of healing the rift between the United States and many European nations that the war caused.
"We really need to support and develop a culture of union and cohesion and certainly not nurture the culture of division," he said. "Selfishness, narcissism and division shall never win."
The visit by Berlusconi on Sunday and Monday gave the president a chance to show the world that not all of Europe is cool to his policies — that trans-Atlantic relations remain strong even though France and Germany didn't back the war effort.
For Berlusconi, the stay here was a reward from Mr. Bush for joining with Britain and Spain in support of the war. It also was an opportunity for Berlusconi, the current president of the European Union, to say that France and Germany aren't the continent's only powers.
In the run-up to the war, Berlusconi helped lobby European leaders for support.
Now America is looking to Europe again for support to end what Army Gen. John Abizaid, the new overall commander of the Iraq operation, is calling "guerrilla-type" warfare waged by remnants of ousted President Saddam Hussein's Baathist Party and fundamentalists possibly tied to terrorists.