Americans were being evacuated Thursday from a major Iraqi air base as Al Qaeda-aligned militants toppled cities in the country's north and threatened to advance toward Baghdad.

A senior U.S. official confirmed to Fox News that Americans were being evacuated from a base in Balad, which had been one of the largest training missions in Iraq.

The three planeloads of Americans are mostly contractors and civilians. The State Department said Thursday that the U.S. Embassy in Baghdad is operating as usual.

But the evacuation means that the vital training mission at Balad, about an hour northwest of Baghdad, has been suspended indefinitely -- despite repeated administration statements that it would continue to support Iraq's military.

Regarding those assurances, one U.S. official clarified to Fox News: "At the same time, we are not going to do anything stupid."

The development signals the worsening security environment in the northern part of the country. One senior official told Fox News that the focus for evacuation at this point is on people outside of Baghdad.

Two senior intelligence sources, though, told Fox News there is serious concern about how to evacuate other Americans out of Iraq if the situation further deteriorates.

"We need places to land, we need safe and secure airfields," one source said, noting that the militants are "seizing airfields and they have surface-to-air missiles, which very clearly threatens our pilots and planes if we do go into evacuation mode."

Sources said "all western diplomats in Iraq are in trouble," and American allies are scrambling to put together an evacuation plan. Military officials said there are "not a lot of good options."

The Obama administration is still trying to determine how to assist the Nouri al-Maliki government, while making clear it does not want U.S. troops in the middle of the fight.

"We are not contemplating ground troops," White House Press Secretary Jay Carney said Thursday.

According to the White House, Vice President Biden spoke Thursday with Maliki and expressed "solidarity" with the Iraqi government in its fight against the Islamic State of Iraq and Syria.

Obama promised Thursday to send more military aid, without saying what kind of new assistance would be given to Baghdad. Two U.S. officials who are familiar with ongoing negotiations told The Associated Press the White House is considering air strikes and increased surveillance, requested this week by the Iraqi defense minister, as the insurgency nears Baghdad.

The Iraqi government has been asking for more than a year for surveillance and armed drones to combat a Sunni insurgency that has gained strength from battlefield successes in neighboring Syria.

Republican lawmakers were harshly critical Thursday of the administration's response. Sen. John McCain, R-Ariz., called for Obama to replace his national security team.

House Speaker John Boehner snapped: "What's the president doing? Taking a nap."

Obama commented on the violence shortly afterward.

"What we've seen over the last couple of days indicates the degree to which Iraq is going to need more help," Obama said. "It's going to need more help from us, and it's going to need more help from the international community."

Several thousand Americans remain in Iraq, mostly contractors who work at the U.S. Embassy in Baghdad on programs to train Iraqi forces on American military equipment like fighter jets and tanks. Those being evacuated from Balad on Thursday included 12 U.S. government officials and military personnel who have been training Iraqi forces to use fighter jets and surveillance drones.

Other U.S. contractors are at a tank training ground in the city of Taji, just north of the capital, that is still in operation for now.

In addition to the possible military assistance, State Department spokeswoman Psaki said the U.S. is sending about $12 million in humanitarian aid to help nearly a million Iraqis who have been forced from their homes by recent fighting in the nation's north and west.

Fox News' Justin Fishel and Adam Housley and The Associated Press contributed to this report.