A squad of American soldiers jog into view and fan out, rifles raised, on the Iraq side of a 100-yard trash-strewn no-man's land separating the Syrian and Iraqi border gates.
Col. Ali Shammar, the Syrian customs officer in charge of this remote desert border post, is visibly agitated by the presence of the nearby soldiers.
"Be careful, do not go closer. They will shoot at us," he tells a group of reporters on a rare visit to the frontier post.
Colonel Shammar has good reason to be alarmed. This desolate border has become a simmering frontline in the violent conflict in Iraq.
American and Iraqi officials routinely accuse Syria of failing to take adequate action to prevent militants from entering Iraq. They say the Syrians are doing little to stop former Iraqi Baathist officials from directing and funding the insurgency from their haven in Damascus.
In a press conference Monday, when asked about foreign fighters entering Iraq through Syria, President Bush said that countries bordering Iraq should "respect the political process" there.
Last week, Bush warned Syria and Iran that "that meddling in the internal affairs of Iraq is not in their interest." His comments followed strong criticism by Iraqi Interim Defense Minister Hazem Shaalan, who said that "terrorism in Iraq is orchestrated by Iranian intelligence, Syrian intelligence, and Saddam loyalists."
But the influence of foreign fighters on the insurgency may have been exaggerated, given the small numbers of Arab volunteers who have been captured or killed in Iraq. And of Iraq's four Arab neighbors, only Syria is regularly singled out for criticism, even though fighters are suspected to have entered from Jordan, Kuwait, and Saudi Arabia.
"It's been blown out of proportion," says a European diplomat in Damascus. "There are individuals helping people enter Iraq but I don't think it is at a strategic level with the knowledge and assistance of the government, and I don't think it's very well organized."
A mile to the north of the border crossing, the Euphrates River winds through the desert, its course marked by a two-mile swath of farmland, palm trees, and bull rushes. On either side of the Euphrates valley lies the desert running from the Turkish border in the north to Jordan in the south - 400 miles of drab wilderness, the border marked only by a berm of bulldozed sand.
In response to US and Iraqi pressure, the Syrian authorities since September have raised and strengthened the berm and added rows of barbed wire and in some places flood lights.
But securing the border completely is nearly impossible. The border police are underfunded and lack equipment and training. Furthermore, the anticipated cooperation on intelligence-sharing between Syrian and Iraqi border authorities has not materialized, diplomats say.
"To be honest it's not just the Syrians' fault. The Syrians are relatively well organized, unlike the Iraqis," says a Western diplomat in Damascus.
Iraqi border posts are routinely attacked by insurgents. Many of the positions have been abandoned by their Iraqi defenders and burned down. Syrian officials say they are doing what they can to secure the border but insist it is impossible to block all infiltrators.
"Committees have been set up with Iraq to address this issue. We are playing a constructive role toward the stability and security of Iraq. Iraq's stability is in our interest," says Mehdi Dakhlallah, Syria's information minister.
As well as tightening border security, Syrian authorities appear to be cracking down on Islamists connected to the insurgency in Iraq. Sixteen Sunni clerics were arrested two weeks ago for mobilizing recruits. Several Islamist fighters who fought in Iraq and returned home have been arrested.
Conservative Sunni clerics continue to condemn American policy in Iraq, but are careful not to be seen encouraging volunteers for the insurgency. "I am stopping people going to Iraq because the situation is chaotic," says Sheikh Wehbi Zuleimi, an expert in Islamic jurisprudence. "I cannot issue a fatwa [religious edict] for people to fight in an atmosphere of chaos."
Normally jammed with a long lines of vehicles, this desolate border crossing was closed six weeks ago prior to the US military assault on Fallujah.
"Our side of the border is open 24 hours a day, but the Americans turn everyone back," says Colonel Shammar.
Rahil Ibrahim, an Iraqi from the border town of Al-Qaim, has spent the past six weeks trying to return home. "I can't see any Americans on the other side to talk to, but the Syrians tell me if I try to cross, the Americans will shoot me."
When does the colonel think the border will reopen? "It's up to God and up to them," he says, pointing at the American soldiers.
The US base regularly comes under mortar and rocket fire from Iraqi rebels. US soldiers often respond by sweeping the area with machine-gun fire, including the Syrian side of the border.
A corner of the customs building facing the American base is peppered with bullet holes. "The Americans fire randomly," says Shammar. "Their bullets once set fire to our kitchen."
Sometimes rockets fly over the American position and explode on the Syrian side of the border. One mortar round impacted beside the customs building. Another blew apart a section of the roof.
The tiny farming hamlet of Hari lies next to the border crossing. Hassan Qabbor points to the spot on the roof of his home where his 16-year-old son, Mari, was killed seven months ago. "He was fetching water from the tank when the Americans shot him," he says.
Mr. Qabbor says that the fatal shots were fired from a watchtower manned by US soldiers. His claims - which could not be verified - have resulted in a lawsuit against the US government in a Damascus court.