Iran's continued efforts to prop up Syrian president Bashar Assad's regime is generating concern inside the Pentagon and raising tensions among U.S. allies in the region.
"We have reason to believe Iran continues to assist [the] Assad regime," Defense Department spokesman Capt. John Kirby told reporters on Thursday, during a briefing at the Pentagon. "That needs to stop."
News reports on Wednesday said Tehran attempted to smuggle a large arms shipment into Syria aboard an Iranian commercial airliner headed to Damascus.
The weapons were allegedly destined for Syrian government forces under the orders of senior commanders in Iran's elite Revolutionary Guard. German news outlet ZDF first reported news of the weapons shipment.
In the past, Tehran has been known to use passenger airliners to move weapons and materiel to Hezbollah fighters in Lebanon.
Kirby declined to confirm the alleged arms shipment from Iran during Thursday's briefing. However, he did note such actions remain "a matter of deep concern [to the United States] and other [allies] share that concern."
White House spokesman Jay Carney accused Tehran of engaging in "malignant behavior" in its attempts to prolong Assad's brutal assault on the Syrian people.
"That fact further highlights Iran's continued effort to expand its nefarious influence in the region, and underscores Iran's fear of a Syria without the Assad regime," he told reporters at the White House on Thursday.
However, the Obama administration has remained committed to its stance of providing only non-lethal support to Syrian rebels, despite news of Iran's increasing role in the conflict.
But the rising level of violence in Syria, punctuated by the massacre of over 100 civilians by government troops in Houla last Saturday, has only increased calls for military action in the country.
GOP presidential nominee Mitt Romney has openly called for arming Syrian rebels and for the White House to take action against countries supplying Assad's forces with weapons.
News reports last week claimed the White House has ordered U.S. officials to begin vetting elements of the Free Syrian Army (FSA), the largest anti-Assad force in the country, to evaluate whether they meet the criteria to receive military support.
The vetting process, first reported by the Associated Press, will likely be run by the Pentagon, State Department and the U.S. intelligence community, with various Gulf states providing the actual weapons to the FSA and others.
Kirby declined to comment on whether the Pentagon was preparing to evaluate or already was evaluating FSA units as part of the White House plan.
However, he did note that certain Gulf states have already begun to funnel weapons into Syria to support the rebellion.
Members of Iran's Revolutionary Guard have been accused by top American commanders of supplying arms to insurgent forces during the darkest days of the Iraq War.
At that time members of Iran's Quods Force, an elite unit within the Revolutionary Guard, reportedly delivered a deadly variant of the roadside bombs, favored by terror groups like al Qaeda and the Taliban, to Iraqi insurgents.
The bomb, known as an explosively formed penetrator (EFP), was designed to penetrate the soft underbellies of American armored vehicles patrolling Iraq at the time.
The weapon resulted in numerous U.S. casualties and forced the Pentagon to develop a new armored vehicle, specifically designed to withstand a blast from an EFP.
This story was updated at 5:31pm to include comments from White House spokesman Jay Carney