U.S. airstrikes "are not going to save" the key Syrian city of Kobani from being overtaken by ISIS, said Pentagon spokesman Rear Adm. John Kirby.
"I think we all should be steeling ourselves for that eventuality," he told reporters in a daily briefing Wednesday.
"We are doing everything we can to halt" ISIS' progress against the town, but airstrikes alone cannot stop the Islamist militants, Kirby added.
"We've been very honest about the limits of air power here. The ground forces that matter the most are indigenous ground forces, and we don't have a willing, capable, effective partner on the ground inside Syria right now - it's just a fact," he said.
The greater U.S. strategy, Kirby said, is to degrade ISIS' ability to sustain itself.
Several senior U.S. administration officials said Kobani will soon fall to ISIS, which calls itself the Islamic State.
They downplayed the importance of it, saying the city is not a major U.S. concern.
But a look at the city shows why it would mark an important strategic victory for the militants. ISIS would control a complete swath of land between its self-declared capital of Raqqa, Syria, and Turkey - a stretch of more than 100 kilometers (62 miles).
As Time.com put it, "If the ISIS militants take control of Kobani, they will have a huge strategic corridor along the Turkish border, linking with the terrorist group's positions in Aleppo to the west and Raqqa to the east."
Staffan de Mistura, U.N. special envoy for Syria, warned of the horrors ISIS could carry out against the people of Kobani - horrors it has carried out elsewhere. "The international community needs to defend them," he said. "The international community cannot sustain another city falling under ISIS."
The terrorist group claimed it had downed at Iraqi army helicopter in Baiji. Photographs posted to an ISIS website show smoke and fire around an aircraft, which is then seen completely charred on the ground.
A truck bomb driven by ISIS exploded near the center of Kobani. Two civilians and a fighter inside the city described it as huge. The target was a security forces building, they said.
However, Kurdish official Idriss Nassan told CNN, the truck did not reach its intended target and detonated early.
See more on this developing story on CNN.com.
The United States is doing what it must to "take the fight to terrorists," leading a coalition of Arab nations in a series of airstrikes against the so-called Islamic State terror group in Syria, U.S. President Barack Obama said Tuesday.
At the same time, the United States took action - on its own - against another terrorist organization, the Khorasan Group. Obama described its members as "seasoned al Qaeda operatives in Syria."
U.S. officials said the group was plotting attacks against the United States and other Western targets.
The plots against the United States were discovered by the intelligence community in the past week, an intelligence source with knowledge of the matter told CNN. The source did not say what the target may have been, but said the plot potentially involved a bomb made of a nonmetallic device like a toothpaste container or clothes dipped in explosive material.
A plot involving concealed bombs on airplanes "was just one option they were looking at," a U.S. official said.
"Once again, it must be clear to anyone who would plot against America and try to do Americans harm that we will not tolerate safe havens for terrorists who threaten our people," Obama said in televised remarks from the White House.
Concern over a possible backlash by the terror groups has prompted the U.S. Department of Homeland Security to issue a bulletin warning law enforcement agencies to be on heightened alert for lone-wolf terror attacks on U.S. soil in wake of the airstrikes, a U.S. law enforcement official with knowledge of the warning told CNN.
The bulletin calls for vigilance as well as scrutinizing social media for anyone encouraging violence in response to the strikes, according to a U.S. law enforcement official with knowledge of the warning's contents. It points to the use of social media as a tactic by ISIS to spread its message and call for violence.
It also advises agencies to look for changes in appearance or behavior in those they're tracking, the official said.
See more on this developing story on CNN.com
Russia is expected to face new sanctions Monday for its actions in Ukraine, President Barack Obama told reporters in Manila, Philippines.
"The sanctions build on the ones that were already in place. We're moving forward with expanded list of individuals," he said.
The move, Obama said, was to spur Russian President Vladmir Putin to "walk the walk not just talk to talk" in resolving the crisis in Ukraine.
If the latest round of sanctions don't work, the next phase could target sectors like banking, Obama said.
The European Union also is expected to impose sanctions Monday on about 15 Russian officials who are believed to be undermining democracy and creating chaos in Ukraine, according to Western diplomats.
The sanctions will include asset freezes and travel bans.
MORE on CNN.com.