Already at the U.S. Embassy in Baghdad, dozens of Marines and Army troops have moved in to beef up security.
The aircraft carrier George H.W. Bush and five other warships are now in the Persian Gulf. More than 500 Marines and dozens of helicopters are on standby.
A top priority: evacuate all Americans at the embassy if it comes to that.
On Monday, Secretary of State John Kerry, in an interview with Yahoo! News, acknowledged that airstrikes on Iraqi targets are under consideration.
"Well, they are not the whole answer, but they may well be one of the options that are important to be able to stem the tide and stop the movement of people who are moving around in open convoys and trucks and terrorizing people," he said. "When you have people murdering, assassinating in these mass massacres, you have to stop that and you do what you need to do."
An unpalatable option
Among the options may be the politically unpalatable one of cooperating with Iran to stop militant gains.
While Kerry didn't say that cooperation with Iran is under active discussion, he wouldn't "rule out anything that would be constructive to providing real stability."
"I think we are open to any constructive process here that could minimize the violence, hold Iraq together -- the integrity of the country -- and eliminate the presence of outside terrorist forces that are ripping it apart," Kerry said.
His comments are the first time such a high-ranking U.S. official has made such a public statement since militants from the Islamic State in Iraq and Syria, or ISIS, began an offensive that has seen vast swaths of northern Iraq fall out of government hands.
Iran plays a key role. It's an ally of Iraq's Shiite-led government. The ISIS militants are Sunni.
A senior security official in Baghdad told CNN last week that the country had sent about 500 Revolutionary Guard troops to help fight the ISIS militants. Iranian President Hassan Rouhani denied the report over the weekend, but said he would be open to helping if asked.
Given the forces at play, direct talks with Iranian officials are something some analysts favor.
"If we engage in a military action without a political solution, we will be seen as backing (Iraqi President Nuri al-) Maliki in a Sunni-Shia civil war. And that is exactly the opposite of what we want to do," said Col. Peter Mansoor, retired.
Tough choices
The United States and Iran held "very brief discussions" about Iraq and the threat posed by ISIS in Vienna, Austria, on Monday, State Department spokeswoman Marie Harf said.
Deputy Secretary of State William Burns is in Vienna for nuclear talks with Iran.
The Obama administration faces some tough choices should it opt to respond.
The U.S. has increased drone surveillance over western and northern Iraq in an effort to gather more intelligence. But using fighter jets to strike at militant targets could be difficult.
"It's the same reason it was so hard to target them when we had 160,000 troops there: They intermingle with the people," said retired Lt. Gen. Mark Hertling. "I think the uninitiated might say: 'Hey, What's so hard about that? Let's just drop a couple bombs.' "
Obama has ruled out U.S. troops on the ground -- a stance to which U.S. Sen. John McCain of Arizona has taken exception.
Approximately a dozen forward air controllers would be needed on the ground before any airstrike, the senator said.
"You need to have people identify the targets in order to really be effective. Remember we're not talking about bombing in towns and cities. There are long stretches of that desert that they have to travel across in their vehicles with their guns on them," McCain told reporters Monday on Capitol Hill.
U.N. Secretary-General Ban Ki-moon told reporters Tuesday in Geneva, Switzerland, that he was "deeply concerned" about the rapidly deteriorating situation in Iraq and reports of mass summary executions by ISIS militants.
"There is a real risk of further sectarian violence on a massive scale within Iraq and beyond its borders," he said.
"I encourage all Iraqi leaders -- political, military, religious and community -- to ensure that their followers avoid acts of reprisal and come together in an inclusive spirit to address this serious threat to the country."
Brutal images
As ISIS continues its violent march, capturing cities along the way, many fear its fighters may reach the capital, Baghdad.
The group, also known as the Islamic State in Iraq and the Levant, has racked up several victories across Iraq in recent days, including the northwestern city of Tal Afar, two villages in Diyala province, and Iraq's second-largest city, Mosul.
Hundreds of thousands have fled. And the crisis is now so dire that the government is pleading for volunteers to help fight the militants.
On Monday, a video surfaced that appeared to show a man in an Iraqi military uniform after he was shot in the face.
A militant fighter boasted on Facebook that he was proud of killing the man and said the video should be passed around to show what could happen to other Shiites.
The new images follow others apparently posted by ISIS to jihadi Internet forums appearing to show the executions of Iraqi security forces and a tweet, on what was claimed to be an ISIS account, saying its members had killed at least 1,700 Shiites.
