Concern: A Syrian soldier looks across at Iraqi border

Major General Amid Suleiman, a Syrian officer, said that American cross-border attacks into Syria had killed at least two border guards, wounded several more and prompted an official complaint to the American embassy in Damascus.

He made the allegations during an official press tour of Syrian security forces on the Iraqi border, which the US claims is a barely guarded passage into Iraq for hardcore foreign jihadis.

While showing off what he said were beefed-up Syrian border measures designed to blunt those criticisms, including new police stations and checkpoints, Maj Gen Suleiman alleged that his own border forces had come under repeated American attack.

"Incidents have taken place with casualties on my surveillance troops," he said, near the Euphrates river border crossing between Syria and Iraq. "Many US projectiles have landed here. In this area alone, two soldiers and two civilians have been killed by the American attacks."

The charge follows leaks in Washington that the US has already engaged in military raids into Syria and is contemplating launching special forces operations on Syrian soil to eliminate insurgent networks before they reach Iraq.

"No one in the administration has any problem with acting tough on Syria; it is the one thing they all agree on," said Edward Walker, a former US ambassador to Egypt and Israel, who is now head of the Middle East Institute think-tank. "I've heard there have been some cross-border activities, and it certainly makes sense as a warning to Syria that if they don't take care of the problem the US will step up itself."

But he warned that the increased blurring of battle lines between Iraq and Syria could turn a diplomatic stand-off between the two nations, playing out at the UN, into a fully fledged military confrontation. "It could escalate. With Syrian border guards getting shot, it could turn into a major issue."

In the Euphrates valley, however, the alleged cross-border fire fights are already a major issue. The Syrian military said that in May, in the divided village of Baghouz, which straddles the Syria-Iraq border about 350 miles north east of Damascus, Abdullah al-Hassake was manning a rundown concrete frontier outpost when he and fellow soldiers heard US helicopters.

He went on to the police station roof to survey the impending battle between US troops and Iraqi insurgents, who flee to the border when under attack, and was killed by fire from the US helicopters.

Syrian officials said that US charges that they were not doing enough to prevent insurgents crossing into Iraq are unfair. They pointed to new barbed wire and reinforced sand barriers across the 400-mile border, which cost £1.5 million, and claimed that they had deported or arrested about 1,500 foreign fighters heading to Iraq.

Much of the border is impossible to seal. Across the divide, the continuing violence in Iraq is all too evident. Both sides have strong ties with the regime of the former Iraqi dictator Saddam Hussein. "The people here are happy to help fighters go to face the Americans," said one local. But reinforced security on the Syrian side had made life harder, he added. That view is supported by some Western diplomats in Damascus, although US defence officials remain sceptical.

"The Syrians have stopped actively encouraging jihadis to go," said one diplomat. "In fact recently they've tried quite hard to stop it."

Across the Euphrates, the border appears to be the likely stage for a future showdown between the US and Syria.

"Sometimes the US soldiers fire at us every day," said Ibrahim Brahim, a Syrian security official. "Sometimes it's simply a mistake, but sometimes it's not. Mostly the US army wants to show us its power."