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.