The Pentagon Thursday said Islamic militants in Iraq might have captured U.S. equipment and vehicles as an al-Qaeda-linked group advanced toward Baghdad.

Members of the Islamic State of Iraq and Syria (ISIS), which has seized control of two major cities, have been posting pictures on Twitter, claiming to have taken equipment given or sold by the U.S. to the Iraqi army.

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A defense official said claims that the group had captured a U.S. Army Blackhawk helicopter were wrong, but that Humvees and armored vehicles might have fallen into the militants' hands.

"I don't have any indication that the reports of them getting Humvees from Mosul are false," said the official.

"It's something that we are consulting the Iraqi government on, as part of our overall consultations with them to get a clear picture of what's happening," the official added.

The advance by ISIS has sparked worries in Washington about the stability of Iraq, and lawmakers Thursday urged the Obama administration to do more to help the government of Prime Minister Nouri al-Maliki.

President Obama said he was considering all options, but White House press secretary Jay Carney later ruled out ground troops.

The chaos there has also raised concerns over future deliveries of U.S. equipment to Iraq under the Foreign Military Financing and Foreign Military Sales programs.

"Ensuring the security of such advanced, sensitive platforms is front and center among our concerns," the defense official said.

Pentagon spokesman Army Col. Steve Warren on Thursday would not say if defense officials are reconsidering future deliveries but said they would work to ensure the weapons were secure.

"We are continuing the process of providing weapons and equipment to the Iraqis so they can counter this ISIS threat," he said.

"Security is part of the process of transferring weapons systems through the FMF and FMS programs," Warren added.

Earlier this year, the U.S. began to expedite the sale of military equipment to Iraq, including 500 Hellfire missiles, 24 Apache helicopters, F-16 fighter jets and other weapons after ISIS took over Fallujah.

The U.S. has also provided armed reconnaissance helicopters, helicopter-fired rockets, sniper rifles and M-16 and M-4 assault rifles to the Iraqi military.

The Pentagon recently notified Congress that it plans to sell 200 Humvees to Iraq for $1 billion.

"We have one of the largest FMF and FMS programs [with Iraq], [which] includes approximately $15 billion worth of equipment and training," Warren said.

"We've sent them, recently, 300 Hellfire missiles, millions of rounds of small arms, thousands of rounds of tank ammunition, and 10 Scan Eagle surveillance [drones] are on schedule for delivery for later in the year," he added

Additional Hellfire missiles and Apache helicopters are scheduled to arrive sometime this summer.

The U.S. is also working to strengthen Iraq's military by offering a second round of 30-day counterterrorism training by U.S. special operations forces in Jordan.

There are currently fewer than 200 Pentagon personnel in Iraq, including military personnel to provide U.S. Embassy security, Warren said.

Deputy Assistant Secretary of State Brett McGurk is currently in Iraq consulting with Iraqi officials.