American warplanes pounded Islamic State positions in Syria for a second day at a strategic post on the Iraqi border, but the campaign did nothing to halt the fighters' advance on a Kurdish town where refugees are fleeing.
Syrian Kurds said Islamic State had responded to U.S. attacks by sending more tanks and fighters into an assault near the Turkish border in the north, where nearly 140,000 civilians have fled in recent days in the fastest exodus yet.
The advance on the town of Kobani is a reminder of the difficulty Washington is likely to face in defeating fighters in Syria, where it lacks strong military allies on the ground. Fighting between Islamic State militants and Kurds could be seen from across the border in Turkey, where the sounds of sporadic artillery and gunfire echoed around the hills.
Washington and Arab allies killed scores of Islamic State fighters in the first 24 hours of air strikes, the first direct U.S. foray into Syria two weeks after President Barack Obama pledged to hit the group on both sides of the Iraq-Syria border.
The initial days of strikes suggest one U.S. aim is to hamper Islamic State's ability to operate across the Iraqi-Syrian frontier. On Wednesday U.S.-led forces hit at least 13 targets in and around Albu Kamal, one of the main border crossings between Iraq and Syria, after striking 22 targets there on Tuesday, said the Syrian Observatory for Human Rights, a body which monitors the conflict in Syria.
The U.S. military confirmed that it had struck inside Syria northwest of al Qaim, the Iraqi town at the Albu Kamal border crossing. It also struck inside Iraq west of Baghdad and near the Iraqi Kurdish capital Erbil on Wednesday.
Perched on the main Euphrates River valley highway, Albu Kamal controls the route from Islamic State's de facto capital Raqqa in Syria to the frontlines in western Iraq and down the Euphrates to the western and southern outskirts of Baghdad.
Islamic State's ability to move fighters and weapons between Syria and Iraq has provided an important tactical advantage for the group in both countries: fighters sweeping in from Syria helped capture much of northern Iraq in June, and weapons they seized and sent back to Syria helped them in battle there.
An Islamist fighter in Albu Kamal area said there had been at least nine strikes on Wednesday by "crusader forces" that had hit targets including in an industrial area.
Damascus says campaign goes "in right direction"
The campaign has blurred the traditional lines of Middle East alliances, pitting a U.S. coalition comprised of countries opposed to Syrian President Bashar al-Assad against fighters that form the most powerful opposition to Assad on the ground.
The attacks have encountered no objection, and even signs of approval, from Assad's Syrian government. Syrian state TV led its news broadcast with Wednesday's air strikes on the border with Iraq, saying "the USA and its partners" had launched raids against "the terrorist organisation Islamic State"
U.S. officials say they informed both Assad and his main ally Iran in advance of their intention to strike but did not coordinate with them.
Jordan, Bahrain, the United Arab Emirates, Qatar and Saudi Arabia have joined in the strikes. All are ruled by Sunni Muslims and are staunch opponents of Assad, a member of a Shi'ite-derived sect, and his main regional ally, Shi'ite Iran.
But some of Assad's opponents fear the Syrian leader could exploit the U.S. military campaign to rehabilitate himself in the eyes of Western countries that had previously demanded he be removed from power.
In perhaps the strongest signal yet that Damascus wants to be seen as fighting the same battle as Washington, Syria's minister for national reconciliation, Ali Haidar, told Reuters: "What has happened so far is proceeding in the right direction in terms of informing the Syrian government and by not targeting Syrian military installations and not targeting civilians."
Damascus was watching the events with caution, Haidar said.
Islamic State advances on Kurds
Even as their outposts elsewhere have been struck, Islamic State fighters have accelerated their campaign to capture Kobani, a Kurdish city on the border with Turkey. Nearly 140,000 Syrian Kurds have fled into Turkey since last week, the fastest exodus of the entire three-year civil war.
Ocalan Iso, deputy leader of the Kurdish forces defending Kobani, said more Islamic State fighters and tanks had arrived in the area since the coalition began air strikes on the group.
"Kobani is in danger," he said, repeating calls for the coalition to expand its air strikes to Islamic State positions near the town. The Syrian Observatory reported air strikes overnight near Kobani. However the U.S. military, Kurdish and Syrian officials did not report strikes in that area.
Iso said Islamic State fighters had advanced to within 8 km from the southern periphery of Kobani - closer than they have been at any stage in the latest offensive.
A group of several dozen Syrian Kurds who had fled the fighting watched from a hillside on the Turkish side of the border as Kurdish fighters battled Islamic State militants in a cluster of villages called Siftek.
The Kurds appeared to be firing mortars from the back of a truck into a village where Islamic State had taken up positions.
"There are more and more Islamic State fighters in the last two days, they have brought all their forces here," said Ahmed Hassan, 60, a Syrian Kurd who fled to Turkey with his family.
"They have heavy weapons. We are running away from them. YPG haven't got heavy weapons, that's why we need help," he said referring to the main Kurdish armed group.
The campaign against Islamic State marks the first direct involvement of the United States in Syria's conflict, which started as a peaceful protest movement in 2011 but degenerated into civil war after a government crackdown.
More than 190,000 people have been killed in the fighting and millions have fled their homes. Gun battles, bombings, shelling and air strikes regularly kill over 150 people a day.
Huge swathes of Syria are now controlled by militias who have fought both Assad and one another. Assad has meanwhile been aided by Shi'ite fighters from Lebanon and Iraq.
Only a year ago, Washington was on the verge of bombing the Syrian government over the use of chemical weapons, until Obama cancelled the strikes at the last minute.
The United States has since turned its attention to Islamic State, which seized northern Iraq's largest city Mosul in June.
Washington launched strikes against the group in Iraq in August. Islamic State responded by beheading two American journalists and a British aid worker, giving rise to more vocal calls for action against it on both sides of the border.