The US and UK have warned that air strikes alone will not prevent Islamic State (IS) fighters from seizing the Syrian town of Kobane.
A Pentagon spokesman said the US and its allies were "doing everything we can from the air" but there were limits to what the campaign could achieve.
Similar views were expressed by British Foreign Secretary Philip Hammond.
A Kurdish leader in Kobane told Reuters news agency IS militants had entered parts of the city amid heavy fighting.
Seizing the town 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.
'No effective partner'
"Air strikes alone are not going to save the town of Kobane," Pentagon spokesman Rear Adm John Kirby said. "We know that. And we've been saying that over and over again."
He said that ultimately rebel fighters in Syria and Iraqi troops would have to defeat IS militants, but it would take time.
"We don't have a willing, capable, effective partner on the ground inside Syria right now," he said, warning that other towns could also fall to the IS.
Likewise Mr Hammond said that it was "never envisaged" that the use of air power "in this battle would turn the tide in the short-term".
"I don't want to suggest that there is anything readily that the coalition can do that will make a fundamental difference (...) in the tactical situation that's faced around Kobane," he said.
Asya Abdullah, a co-leader of the Kurdish Democratic Union Party representing Syrian Kurds in Kobane, said that on Wednesday night IS entered two districts of Kobane with heavy weapons, including tanks.
"Civilians may have died because there are very intense clashes," he said. 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 about 100m towards the town centre on Wednesday evening.
It added that IS was bringing in reinforcements from its stronghold in Raqqa province.
The US Central Command said in a statement late on Wednesday that eight coalition air strikes had hit targets in Kobane. It said five IS armed vehicles, an IS supply depot and other buildings had been destroyed.
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."
Meanwhile Turkey remains under intense pressure to do more to help the Kurdish forces in Kobane.
In a BBC interview a senior Kurdish politician in Turkey, Meral Daniss Bestas, called for "a safe corridor for Kurds to supply arms and humanitarian aid to Kobane".
At least 19 people have been killed in Kurdish protests over Turkey's role.
Kurds are angry that Turkey has prevented Kurdish fighters crossing the border to battle IS in Kobane and some protesters want it to take military action against IS.
Fighting between police and demonstrators continued in Istanbul throughout Wednesday night, with protesters congregating in mostly Kurdish districts.
Protesters hurled petrol bombs at police, who responded with water cannon and tear gas.
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.
However the White House has said that it does not support the idea.