WASHINGTON -- The American effort to stem the flow of Iranian arms to Syria has faltered because of Iraq's reluctance to inspect aircraft carrying the weapons through its airspace, U.S. officials say.
The shipments have persisted at a critical time for Syria's President Bashar Assad, who has come under increasing military pressure from rebel fighters. The air corridor over Iraq has emerged as a main supply route for weapons, including rockets, anti-tank missiles, rocket-propelled grenades and mortars.
Iran has an enormous stake in Syria, which is its staunchest Arab ally and has also provided a channel for Iran's support to the Lebanese Islamist movement Hezbollah.
To the disappointment of the Obama administration, American efforts to persuade the Iraqis to conduct random inspections of the flights have been largely unsuccessful.
Secretary of State Hillary Rodham Clinton secured a commitment from Iraq's foreign minister in September that Iraq would inspect flights from Iran to Syria. But the Iraqis have inspected only two, most recently on Oct. 27. No weapons were found, but one of the two planes that landed in Iraq for inspection was on its way back to Iran after delivering its cargo in Syria.
Adding to the United States' frustrations, Iran appears to have been tipped off by Iraqi officials as to when inspections would be conducted, American officials say, citing classified reports by U.S. intelligence analysts.
Iran's continued efforts to aid the Syrian government were described in interviews with a dozen U.S. administration, military and congressional officials, most of whom requested anonymity to discuss intelligence matters.
Iraqi officials insist that they oppose the ferrying of arms through Iraq's airspace. They also cite claims by Iran that it is merely delivering humanitarian aid, and they call the American charges unfounded.
Meanwhile, in Syria itself, rebels and government forces clashed on the outskirts of Damascus, explosions rumbled in the distance and warplanes screeched overhead. The rebels on Saturday appeared to be making their strongest push toward the city since the government repelled an offensive there in July.
A quiet tension prevailed downtown, but security checkpoints were proliferating and there were reports that Mr. Assad was preparing loyal divisions to defend the city, the capital and heart of his power.
Military analysts warned that it was impossible to know whether a decisive battle for Damascus was beginning, especially as Syrians lost access to the Internet for 53 hours, limiting the flow of information, before it was restored Saturday. But they said that a government fight to defend its core in the capital could be the fiercest and most destructive phase yet of the 20-month conflict.
For decades, the Assad family has settled loyal military families, many from its minority Alawite sect, in the western outskirts of Damascus, where the presidential palace sits on a plateau overlooking the city. The current fighting suggested that the government was trying to insulate those areas, along with the city center and airport, from the semicircle of urban sprawl curving from northeast to southwest, where rebels have strengthened their position in recent days, overrunning a string of small bases.