Secretary of State John Kerry met Turkish leaders on Friday to try to win support for U.S.-led military action against Islamic State, but Ankara's reluctance to play a frontline role showed the difficulty of building a coalition for a regional war.

Kerry has been touring the Middle East to build support for President Barack Obama's plan, announced on Wednesday, to strike both sides of the Syrian-Iraqi frontier to defeat Islamic State Sunni fighters that control swathes of both countries.

Turkey, a NATO member which shares long borders with both Syria and Iraq, is one of Washington's main allies in the region but has so far conspicuously avoided committing to the new military campaign.

U.S. officials downplayed hopes of persuading Ankara to take a significant role in any military involvement, saying the talks would focus on issues including Turkey's efforts to stem the flow of foreign fighters crossing its territory and its role in providing humanitarian assistance.

The Ankara meetings came a day after Kerry signed up 10 Arab allies to a "coordinated military campaign" to fight the Sunni militants. But Turkey, which attended Thursday's talks in Saudi Arabia, did not join the Arab states in signing up to the final communique.

A senior Turkish official said Ankara stayed out of the communique in part due to the sensitivity of efforts to free 46 Turkish hostages captured by Islamic State fighters in Iraq in June. But pro-government Turkish media ran articles on Friday expressing broader scepticism over Obama's plans.

U.S. officials emphasized that Turkey could help in other ways, without pledging to join the nascent military coalition.

"The Turks have played an extraordinary role on humanitarian aspects of the situation ... and they are going to play and have been playing a pivotal role in our efforts to crack down on foreign fighter facilitation and counter terrorist finance," a senior U.S. State Department official said ahead of the talks.

"We consider our approach to the stability and security of Syria and Iraq and to the campaign against ISIL to be holistic and include lines of effort well beyond military action," the official said, using the acronym for the Islamic State in Iraq and the Levant, the group's former name.

"NOTORIOUS"

Obama's plan to fight Islamic State simultaneously in Iraq and Syria thrusts the United States directly into the midst of two different wars, in which nearly every country in the region has a stake, alliances have shifted and strategy is dominated by Islam's 1,300-year-old rift between Sunnis and Shi'ites.

Islamic State is made up of Sunni militants, who are fighting against a Shi'ite-led government in Iraq and a government in Syria led by members of a Shi'ite offshoot sect. It also battles against rival Sunni Islamists and more moderate Sunni groups in Syria, and Kurds on both sides of the border.

From the early days of the Syrian conflict, Turkey has backed mainly Sunni rebels fighting against President Bashar al-Assad. Although it is alarmed by Islamic State's rise, it is wary about any military action that might weaken Assad's foes.

It is also concerned about strengthening Kurds in Iraq and Syria. Turkey's own Kurdish militants waged a three-decade insurgency against the Turkish state and are engaged in a delicate peace process.

Pro-government newspapers on Friday welcomed Ankara's reluctance to take a front-line role in the coalition, questioning whether U.S.-led military action was the answer and drawing parallels to 2003, when Turkey's parliament rejected a U.S. request to use Turkish territory to invade Iraq.

"The U.S. administration's air strikes are notorious in terms of the civilian deaths they cause," wrote Hilal Kaplan, a columnist in the pro-government Yeni Safak newspaper.

"To prevent such a strategy from leaving Sunnis who are already fed up with the oppressive Shi'ite hegemony in Iraq more dependant on IS is something that needs to be discussed."

Lebanese Foreign Minister Gebran Bassil told Reuters some Arab states in Jeddah had proposed expanding the campaign to fight other Islamist groups besides Islamic State, a move Turkey would also probably oppose.

Turkey's support for the rebels fighting Assad, including groups that some Western allies balked at backing, has laid it open to accusations it aided radical Islamists and contributed to Islamic State's rise, a notion Ankara strongly rejects.

Francis Ricciardone, who was until late June the U.S. ambassador in Turkey, said on Thursday Ankara had supported groups including the Nusra Front, al Qaeda's Syrian branch.

"We ultimately had no choice but to agree to disagree," Ricciardone told a conference call arranged by the Atlantic Council think-tank on Thursday. "The Turks frankly worked with groups for a period, including al Nusra, whom we finally designated as we're not willing to work with."

(Additional reporting by Jonny Hogg, Humeyra Pamuk and Nick Tattersall; Writing by Nick Tattersall; Editing by Peter Graff)