The US and UK have warned that air strikes alone will not prevent Islamic State (IS) fighters from seizing the strategic 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 said IS had entered two more districts overnight, bringing in heavy weapons.
Seizing the town would give 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 Syrian side of the border.
France has said it supports the idea but the White House said it was "not something that is under consideration right now".
Three weeks of fighting over Kobane have cost the lives of at least 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 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," she said. Another official there said IS had seized some buildings in the east and that there was fierce fighting with Kurdish forces.
The UK-based Syrian Observatory for Human Rights monitoring group said IS had advanced about 100m (320ft) 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 Kurdish forces in Kobane.
Nato Secretary General Jens Stoltenberg and President Obama's envoys to the coalition against IS are in Ankara for talks with President Recep Tayyip Erdogan on possible Turkish action.
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 fight 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 wants a buffer zone along the Turkish border inside Syria - enforced by a no-fly zone - to ensure security and ease the refugee influx into Turkey.
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