On Thursday, the Bush administration released an unclassified version of a national intelligence estimate that supported Lieberman's allegations against Syria. But the report said Damascus, despite its increase in support for militias in Iraq, has cracked down on Al Qaida in wake of a determination that the Islamic movement threatens Syrian national security.

Lieberman said U.S. intelligence has determined that the lion's share of Sunni volunteers fly into Damascus International Airport and then proceed overland to Iraq. He said this makes Damascus the hub of Al Qaida travel in the Middle East.

"There is a feeling that something has to be done," a senior congressional staffer said.

Officials said about 80 volunteers per month have been processed and trained in Syria before being sent to strike Shi'ite and U.S.-led coalition targets in Iraq. They said most of the volunteers, who meet their handlers in Syria, were assigned suicide bombing missions in Iraq.

"Syria refuses to tighten its visa regime for individuals transiting its territory," Lieberman said.

"Syrian President Bashar Al Assad cannot seriously claim that he is incapable of exercising effective control over the main airport in his capital city," Lieberman wrote in the Wall Street Journal. "Syria is a police state, with sprawling domestic intelligence and security services. The notion that Al Qaida recruits are slipping into and through the Damascus airport unbeknownst to the local Mukhabarat [intelligence agency] is totally unbelievable."

The proposed legislation, expected to be introduced in Congress in September, would ban Damascus International Airport to international carriers. The sources said the sanctions could affect the U.S. activities of airlines that violate the proposed sanctions.

Alitalia, Air France and British Airways conduct regular flights to Damascus. U.S. carriers do not fly to Syria.