Kobane: New IS push amid US warning on air strikes
A Pentagon spokesman said it was a reality that "towns and villages - perhaps Kobane - will be taken by IS".
Seizing Kobane would give the IS jihadists full control of a long stretch of the Syrian-Turkish border.
The US also appeared to be at odds with allies over a Turkish idea to create a buffer zone or safe haven along the border.
Three weeks of fighting over Kobane has cost the lives of 400 people, and forced more than 160,000 Syrians to flee across the border to Turkey.'Steeling ourselves'
Asya Abdullah, a co-leader of the Kurdish Democratic Union Party representing Syrian Kurds in Kobane, said: "Tonight [Islamic State] has entered two districts with heavy weapons, including tanks. Civilians may have died because there are very intense clashes."
At the scene: Paul Adams, BBC world affairs correspondent
The images are powerful enough, but the sound is sometimes overwhelming. At times today, it seemed the entire eastern side of Kobane was one vast street battle. It was relentless. Thick clouds of smoke drifted across the town as grenades exploded.
And all day, another series of massive air strikes; each towering black cloud greeted with delighted cheering by Turkish Kurds who have come to watch, with mounting dread, the assault on their Syrian cousins across the fence.
In groups large and small, they gather as close to the fence as they can get, shouting chants of defiance and solidarity. They are furious with Turkey for what they believe is Ankara's complicity in the rise of Islamic State.
Another official there said IS had seized some buildings in the east and that there was fierce fighting with Kurdish resistance forces.
The UK-based Syrian Observatory for Human Rights monitoring group said IS forces had advanced around 100m (yards)towards the town centre on Wednesday evening.
At a news briefing, Pentagon spokesman Rear Adm John Kirby said: "Air strikes alone are not going to do this. They're not going to fix this. They're not going to save the town of Kobane. We know that."
The US is training moderate Syrian rebels in Saudi Arabia to provide a ground force, but this could be months away.
When asked if this meant Syrian towns could fall to IS, he said: "I think we all should be steeling ourselves for that eventuality, yes.
"We all need to prepare ourselves for the reality that other towns and villages and perhaps Kobane will be taken by IS."
Earlier US Secretary of State John Kerry said the US was deeply concerned about the people of Kobane.
But he added: "Horrific as it is to watch the violence, it is important to keep in mind the US strategic objective" - which, he added, was to deprive IS of command-and-control centres and the infrastructure to carry out attacks.
The chairman of the US Joint Chiefs of Staff, Gen Martin Dempsey, told ABC News that IS was becoming "more savvy".
"We have been striking when we can... They don't fly flags and move around in large convoys the way they did. They don't establish headquarters that are visible or identifiable."
The US Central Command said six coalition air strikes had hit targets to the south and south-west of Kobane over Tuesday and Wednesday. It said an armoured personnel carrier, four "armed vehicles" and two artillery pieces were destroyed.
Turkey remains under intense pressure to do more to help the Kurdish forces in Kobane.
At least 19 people have been killed in Kurdish protests over Turkey's role.
Kurds are angry that Turkey has prevented fighters crossing the border to fight IS in Kobane.
Last week Turkey's parliament also authorised military action against the jihadists in Iraq and Syria, but so far no action has been taken.
Turkey insists that a buffer zone be set up on the Turkish border inside Syria, enforced by a no-fly zone to ensure security and ease the refugee influx into Turkey.
France on Wednesday threw its weight behind the buffer zone and UK Foreign Secretary Philip Hammond said he was "exploring" the issue as well.
However, White House spokesman Josh Earnest told reporters: "It's not something that is under consideration right now."