By Jason Szep

PARIS (Reuters) - Several Arab countries have offered to join the United States in air strikes against Islamic State targets, U.S. officials said on Sunday, indicating a possible widening of the air campaign against militants who have seized parts of Iraq and Syria.

The officials declined to identify which countries made the offers. But they said they were under consideration as the United States begins to identify country roles in its emerging coalition against jihadists who have declared a caliphate or Islamic state ruled under Sharia law in the heart of the Middle East.

The addition of Arab fighter jets could strengthen the credibility of the American-led campaign in a region skeptical of how far Washington will commit to a conflict in which nearly every country has a stake, set against the backdrop of Islam's 1,300-year-old rift between Sunnis and Shi'ites.

“I don’t want to leave you with the impression that these Arab members haven’t offered to do air strikes because several of them have,” a senior U.S. State Department official told reporters in Paris.

The official said the offers were not limited to air strikes on Iraq. "Some have indicated for quite a while a willingness to do them elsewhere," the official said. "We have to sort through all of that because you can’t just go and bomb something."

So far, France has been the only country to publicly offer to join U.S. air strikes on Islamic State targets, although limiting these to Iraq. Britain, Washington's main ally in 2003, has sent mixed messages. It has stressed the West should not go over the heads of regional powers nor neglect the importance of forming an inclusive government in Iraq.

The U.S. comments come a day after Islamic State stirred fresh outrage with a video purporting to show the beheading of British aid worker David Haines. British Prime Minister David Cameron called it "a despicable and appalling murder," and vowed to bring the killers to justice.

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U.S. Secretary of State John Kerry will meet British Foreign Secretary Philip Hammond during a conference on Iraq in Paris on Monday. “I am sure that will be a topic of discussion,” a second senior U.S. State Department official said, referring to the beheading.

The conference brings Iraqi authorities together with 15 to 20 international players. It comes ahead of a U.N. Security Council ministerial meeting on Sept. 19 and a heads of state meeting at the U.N. General Assembly later this month.

The U.S. officials spoke on condition of anonymity.

“MORE AGGRESSIVE KINETIC ACTION”

Offers of Arab air participation have been made both to U.S. Central Command (CENTCOM) overseeing the American air campaign and to the Iraqi government, the first U.S. official said.

“I want to be clear that there have been offers both to CENTCOM and to the Iraqis of Arab countries taking more aggressive kinetic action against ISIL,” said the official, referring to the group by its former name Islamic State in Iraq and the Levant.

On Thursday, Kerry won backing for a "coordinated military campaign" from 10 Arab countries - Egypt, Iraq, Jordan, Lebanon and six Gulf states including rich rivals Saudi Arabia and Qatar.

Although President Barack Obama has authorized the use of U.S. fighter jets in a plan announced on Wednesday to attack both sides of the Syrian-Iraqi frontier and defeat Islamic State Sunni fighters, no decision has been made yet on whether to carry them out, U.S. officials said.

As of Saturday, U.S. fighter jets had conducted 160 air strikes on Islamic State positions in Iraq. The United States will present a legal case before expanding them into Syria, U.S. officials said, justifying them largely on the basis of defending Iraq from militants who have taken shelter in neighboring Syria during its three-year civil war.

“If and when there is a decision to actually conduct a strike, as opposed to authorize strikes which is what the president has talked about, I think we’ll be very clear about what the basis of that is going to be, but I wouldn’t want to get ahead of that,” one official said of possible U.S. air strikes on Syria.

“But surely the defense of Iraq and Iraq’s right to self defense from threats and invasion across its border will play a part in that decision.”

An expansion of U.S. air strikes into Syria would deepen a conflict that already cuts across multiple sectarian lines. Islamic State is made up of Sunni militants fighting a Shi'ite-led government in Iraq and a government in Syria led by members of a Shi'ite offshoot sect.

Several Arab states have powerful air forces, including Saudi Arabia and United Arab Emirates.

But in Saudi Arabia, as elsewhere in the region, Kerry has faced reluctance to be seen aggressively joining the U.S. campaign in Iraq and Syria, fearing in some cases reprisals by extremists or forces loyal to the Syrian government.

While Gulf Arab leaders are alarmed at the prospect of a disintegrating Iraq that could shelter Islamist militants who may target their own countries, some such as Saudi Arabia also deeply fear the fight against Islamic State could hasten U.S.-Iranian detente.

Saudi Arabia and Iran back opposing sides in wars and political struggles in Iraq, Syria, Lebanon, Bahrain and Yemen. Saudi Arabia’s ruling Sunni Muslim princes, for instance, see the battle to overthrow Syrian President Bashar al-Assad, an ally of Riyadh's foe Shi'ite Iran, as pivotal to their own future, fearing that if Assad survives, Tehran will expand its influence across the region and entirely encircle the kingdom.

(Additional reporting by John Irish; Editing by Cynthia Osterman)