U.S. aircraft carried out 18 strikes on Islamic State positions near the besieged Syrian border town of Kobani and five strikes against the group in Iraq on Tuesday and Wednesday, the U.S. military's Central Command said.



The planes struck 16 buildings occupied by Islamic State militants and destroyed several of their fighting positions near Kobani, a Kurdish town on the Syrian border with Turkey, it said in a statement on Wednesday.



It said four strikes near Baiji, the site of Iraq's largest oil refinery, destroyed an artillery piece, a Humvee, a machine gun and a building used by the group, which has seized large parts of Iraq and Syria.



Another strike near Haditha Dam in Iraq destroyed an armed vehicle, the statement said.

The U.S. military has named the coalition operation against Islamic State in Iraq and Syria "Inherent Resolve," a U.S. military official said on Wednesday.

Meanwhile, Kurdish forces making a stand against Islamic State fighters in Kobani say they have begun coordinating with the U.S. military to provide targets for air strikes.

A four-week siege of the mainly Kurdish town on the border with Turkey has become a focus of the U.S.-led effort to halt the militants who have seized swathes of territory in Syria and Iraq. The United Nations has warned that a massacre could take place in the town if it falls to militants, who now control nearly half of it after pushing their way inside last week.



A U.S.-led alliance has been bombing Islamic State targets in Iraq since August and extended the campaign to Syria in September. After weeks in which Kobani was rarely targeted, the town has become the main focus of strikes in recent days.



Kurdish officials said the improvement in the effectiveness of air strikes in and around Kobani had come about after the main Kurdish armed group, the YPG, began giving the coordinates of Islamic State positions to the U.S.-led alliance.



"The senior people in YPG tell the coalition the location of ISIL targets and they hit accordingly," Polat Can, a YPG spokesman, told Reuters, using an acronym for Islamic State.

"Some of (the militants) have withdrawn, but they regroup and return. But because the air strikes are working in coordination, they hit their targets well," he said.



He did not disclose how the YPG fighters share coordinate information with the U.S. military.

Tim Ripley, a British defence expert with Jane's Defence Weekly, said U.S. air controllers responsible for picking targets could check any information provided by YPG fighters, by also using spotters watching the fighting from across the frontier in Turkey and video from drones.



It was also possible that U.S. target spotters could be operating alongside the YPG on the ground in Kobani, Ripley said, but he thought this unlikely because of the high risk to the operation if they were injured or captured.



The Kurdish YPG have been struggling to defend Kobani, also known as Ayn al-Arab, from better armed Islamic State fighters who have used tanks, artillery and suicide truck bombs in a month-long offensive against the town at the Turkish border.



Kobani appeared close to falling a week ago as Islamic State fighters entered its eastern and southern districts and flew their black flag. As recently as Saturday, Kurdish leaders were calling for the air strikes to be stepped up. But in recent days, the air strikes have increased and the militants have not made much progress. The Kurds say they have taken back areas on the west of the town.



U.S. President Barack Obama voiced deep concern on Tuesday about the situation in Kobani as well as in Iraq's Anbar province west of Baghdad, which U.S. troops fought to secure during the Iraq war.



The intensified air campaign around Kobani has lifted the spirits of Kurds who have maintained a vigil watching the fighting from a hilltop just over the border in Turkey.



Dozens cheered as a powerful air strike hit eastern Kobani on Wednesday afternoon, sending up a plume of smoke. One of the spectators compared Obama to a prophet.



The Syrian Observatory for Human Rights, which tracks the war using a network of sources on the ground, said one of the allied air strikes in the last day killed a group of Islamic State fighters just 50 meters from a Kurdish position.



Rami Abdulrahman, who runs the Observatory, said seven Islamic State fighters had been killed in clashes with the Kurds on Wednesday, compared to five Kurdish fighters.

"(The air strikes) are more serious than before because the coordination has grown in the last six days," Abdulrahman said.



The town's plight has angered Kurds across the border in Turkey, who accuse the Turkish government of doing too little to help protect their kin in the battle, which has unfolded within view of Turkish tanks at the frontier.



Turkey has taken in 200,000 refugees from the area but has rejected the Syrian Kurds' request to open a land corridor so they can resupply the besieged town with arms and fighters from other parts of northern Syria.



Washington has been cautious about predicting that its air power can make a difference, with U.S. officials saying the town could still fall. The U.S. military says its strategic objective is to degrade Islamic State's capabilities, rather than protect particular towns in what is expected to be a long conflict.