The Syrian Observatory for Human Rights monitoring group said IS rockets hit the town for the first time since the jihadist assault began on September 16, wounding 12 people.
The IS campaign there has already driven 160,000 refugees into Turkey.
Royal Air Force Tornado GR4 jets took off from Britain's RAF Akrotiri on Cyprus for Iraq but returned to base without dropping their laser-guided bombs.
"On this occasion no targets were identified as requiring immediate air attack by our aircraft," said a defence ministry spokesman in London.
Belgium and Denmark have also approved plans to join France and the Netherlands in targeting IS in Iraq, allowing Washington to focus on the more complex operation against its Syria base.
Washington warned that the jihadists could not be defeated in Syria by air power alone, saying that up to 15,000 "moderate" rebels would need to be trained.
In a video posted online, an Al-Nusra Front spokesman threatened the coalition nations.
"These states have committed a horrible act that is going to put them on the list of jihadist targets throughout the world," Abu Firas al-Suri said.
"This is not a war against Al-Nusra, but a war against Islam."
Saturday was the second time US-led air strikes had been reported around Ain al-Arab since the IS advance began.
- Turkey mulls 'necessary steps' -
President Recep Tayyip Erdogan said Turkey could take a military role in the coalition, the Hurriyet daily reported.
He said the government would go to parliament with a motion on October 2, after which "all the necessary steps" would be taken.
Ankara had insisted its hands were tied over dozens of Turkish hostages abducted by IS in Iraq, but they are now free.
Hundreds of Syrian Kurdish refugees, clutching whatever they could grab, crossed the border Saturday to safety.
Turkey's NTV television reported that shells fired from Syria hit Suruc, about 10 kilometres (six miles) north of the border, wounding two women.
Senior Syrian Kurdish official Newaf Khalil told AFP that air strikes hit the IS-held town of Ali Shar east of Ain al-Arab, destroying several IS tanks.
Saturday's air strikes came a day after hundreds of Kurdish fighters crossed from Turkey to reinforce Ain al-Arab's defenders.
Coalition aircraft also pounded the Euphrates valley city of Raqa, which the jihadists have made the headquarters of the "caliphate" they declared in June over swathes of Iraq and Syria.
"At least 31 explosions were heard in Raqa city and its surroundings," said the Britain-based Observatory.
Washington has been keen not to let Syrian President Bashar al-Assad exploit the anti-IS campaign to make gains in the more than three-year-old civil war.
- 'Near continuous' combat sorties -
The US and Arab allies began air strikes against IS in Syria on Tuesday, more than a month after Washington launched its air campaign against the jihadists in Iraq.
Washington had been reluctant to intervene in Syria, but acted after the jihadists captured more territory and committed widespread atrocities, including beheading three Western hostages.
A US defence official told AFP Friday the Syrian mission is now similar to Iraq's, with "near continuous" sorties.
Washington also plans to train and arm 5,000 Syrian rebels, although top US military officer General Martin Dempsey said 12,000-15,000 men would be required to recapture "lost territory" in Syria.
Dempsey said defeating IS would take more than air strikes and that "a ground component" was an important aspect of the campaign.
At the UN General Assembly, Russian Foreign Minister Sergei Lavrov accused Washington of declaring its "right to unilateral use of force anywhere to uphold its own interests," in a veiled reference to the Syrian campaign.
European governments have so far ruled out strikes in Syria, although Britain "reserved the right" to intervene there if there was an imminent "humanitarian catastrophe".
Washington has instead been supported in its Syria campaign by Arab allies Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates.
Iran's ground forces commander General Ahmad Reza Pourdestanahas warned that it too would attack IS in Iraq if it approached the border, state media reported.
The Sunni extremists of IS control territory north of Baghdad, including in Diyala province bordering Shiite Iran.
In Diyala, the army said Shiite militias retook a dam after fighting believed to have killed dozens, security sources said.