On a plateau overlooking the Iraqi desert, Syrian army recruit Ahmad Salem stands by his Russian-made mortar in a newly dug trench.
Salem is one of 7,000 troops manning 560 positions that Syrian officials say have been set up along the 650-km border since the Iraq war.
Washington says Syria is a conduit for Arab militants crossing into Iraq to fight US troops and accuses Damascus of doing too little to control the flow of infiltrators.
Near a post almost 400 km northeast of Damascus, Syrian commanders do not conceal their annoyance at the US accusations, in conversation with reporters who accompanied foreign diplomats on a rare extended tour.
“The responsibility for securing the borders cannot fall on one party, the two sides should be responsible. We have not seen any presence of the Iraqis. Where are they?” said a senior Syrian border guard official, who declined to be named.
He gestured across Syrian sand barriers towards the desert of Iraq's huge Anbar province, which borders Syria and Jordan.
Iraqi border posts have stood deserted since the US-led invasion that toppled Saddam Hussein in 2003.
US Marines frequently mount offensive operations in Anbar, the heart of the Sunni-led insurgency, but lack the troop numbers to remain in areas after they have driven rebels away.