The US-led coalition has carried out its most sustained air attacks so far on Islamic State fighters attacking the Turkey-Syria border town of Kobane.
Syrian Kurdish fighters said the strikes were the most effective yet but should have come much earlier.
Correspondents said the surge of IS appeared to have been halted, although fighting around Kobane continues.
At least 12 people were killed in protests by Kurds in Turkey about the lack of Turkish military support.
The leader of the Syrian Kurdish Democratic Unity Party (PYD) said the situation overnight remained very serious, with fighters from its armed wing, the Popular Protection Units (YPG), under intense pressure.
"There is heavy fighting going on by YPG forces and they're trying to defend the civilians," Salih Muslm said. "There is a very large operation against them."
At least 400 people have died in three weeks of fighting for Kobane, monitors say, and 160,000 Syrians have fled across the border to Turkey.
The latest media release from the US military confirmed five air strikes around Kobane, saying they were on Monday and Tuesday, but without specifying exactly when.
The strikes take place as the Obama administration is reported to be increasingly frustrated by what it sees as Turkey's excuses for not doing more to intervene militarily.
"There's growing angst about Turkey dragging its feet to act to prevent a massacre less than a mile from its border," a senior administration official was quoted by the New York Times as saying.
"After all the fulminating about Syria's humanitarian catastrophe, they're inventing reasons not to act to avoid another catastrophe," the official said.
"This isn't how a Nato ally acts while hell is unfolding a stone's throw from their border."
US Secretary of State John Kerry is also reported by the New York Times to have had "multiple phone calls" with Turkey's prime minister and foreign minister in an effort to resolve the border crisis.
At the scene: BBC's Paul Adams on Syria-Turkey border
Air strikes took centre stage on Tuesday, apparently bringing the IS advance to a juddering halt. With jets overhead for long periods, IS clearly had to spend time under cover to avoid being hit. As a result, there was nothing like the intensity of fighting seen on Monday. At times, Kobane seemed eerily quiet.
Significantly, it seems the Kurdish defenders of Kobane are now communicating directly with US-led coalition forces. But air strikes alone may not be enough to stop IS taking Kobane in the long run. The Kurdish YPG militia claims to have the upper hand in street fighting, but it is outnumbered and outgunned by IS.
Only a ground operation, or significant military assistance from Turkey could carry any guarantee of success, and the prospects for this seem remote. Turkey's conditions are not ones the US seems ready to accommodate.
The US military said the most recent air strikes had destroyed four IS armed vehicles and an "IS unit", and damaged one IS tank and one armoured vehicle.
However, the BBC's Paul Adams, on the Syria-Turkey border, says more air strikes on Tuesday afternoon - bringing the total to eight for the day - were by far the most sustained coalition action in the area.
He says that as a result, fighting in the city died down considerably - it was quiet but for occasional crackles of gunfire in the afternoon.
Speaking on a visit to a refugee camp for Syrians on Tuesday, President Recep Tayyip Erdogan said air power alone could not defeat IS: "We had warned the West. We wanted three things: no-fly zone, a secure zone parallel to that, and the training of moderate Syrian rebels."
He said that "the terror will not be over... unless we co-operate for a ground operation", although he gave no further details.
Across Turkey on Tuesday, Kurds vented their anger at the government's lack of military support for the defenders of Kobane.
Analysis: BBC's Mark Lowen in Istanbul
The crisis in Kobane is reawakening the ghosts of the civil war between Turkey and the Kurds.
While Islamic State tightens its grip on Kobane, Turkey is still holding fire on deploying troops. It remains reluctant to help the Kurdish militia in Syria, which has close links with Kurdish fighters here.
And the Turkish government has again called for the US-led coalition to target the Assad regime as well as IS - and for a no-fly zone to ease the refugee influx into Turkey. But neither goal seems within reach, the US state department reiterating that the air strikes remained focused on IS alone. The Kurds say Turkey's failure to act will lead to the fall of Kobane.
Police used tear gas and water cannon as unrest spread to at least six south-eastern cities.
Dozens of Kurdish demonstrators also smashed a glass door and entered the European Parliament in Brussels.
Hundreds more protesters demonstrated in Berlin and other German cities.
In New York, the UN's special envoy for Syria, Staffan de Mistura, said the Syrian Kurds had defended Kobane with great courage and the international community should now take concrete action to support them, although he did not say whether he thought ground troops should join the fight.