Syrian and other Arab ground forces must be found to take on Islamic State, U.S. Secretary of State John Kerry said on Thursday, saying the militant group would not be defeated by air strikes alone.

Kerry was speaking hours after Britain began bombing Islamic State targets in Syria, joining forces with France and the United States, nearly three weeks after the jihadist group killed 130 people in attacks across Paris.

British Prime Minister David Cameron says there are as many as 70,000 moderate opposition fighters in Syria ready to take on Islamic State with the help of foreign air strikes, an assertion opponents of the bombing campaign have questioned.

Kerry suggested that if a political solution could end the fighting between the government of Syrian President Bashar al-Assad and opposition groups and sufficient ground troops could be mustered, the militant group could be vanquished in months.

"I think we know that without the ability to find some ground forces that are prepared to take on Daesh, this will not be won completely from the air," Kerry said, using an Arabic term for the jihadist group.

Asked later if he meant Western ground forces, Kerry said after a meeting in Belgrade of the Organisation for Security and Cooperation in Europe (OSCE): "(I'm) talking about Syrian and Arab, as we have been consistently."

A U.S. official said Kerry was speaking mainly of Syrian ground forces, but it was conceivable troops of other Arab nations could be involved.

"They have to be troops from those countries (who) know the culture, know the groups, know the terrain," he told reporters on condition of anonymity. "It could possibly include Arab partners but we're not at that stage right now."

It is unclear if any Arab state would contribute significant ground forces to Syria anytime soon because of disagreements about the war and the fact that many militaries are already stretched tackling insurgencies, protecting borders or fighting conflicts closer to home.

In a policy reversal, the United States on Oct. 30 said it would send up to 50 U.S. special forces to Syria to coordinate on the ground with U.S.-backed rebels.

Kerry met his Russian counterpart, Sergei Lavrov, on the sidelines of the OSCE meeting, with Washington and Moscow at odds over the fate of Assad more than four years into a war that has killed over 250,000 people.

The West says Assad must go, but Russia launched its own air strikes on Sept. 30 in support of his government, saying it was going after Islamic State. Western officials say Russian jets have hit mainly other anti-Assad rebels.

Kerry said a "political transition" in Syria could pave the way for a united front against Islamic State - "the Syrian army together with the opposition ... together with Russia, the United States and others to go and fight Daesh".

"Just imagine how quickly this scourge could be eliminated, in a matter of literally months, if we were able to secure that kind of political resolution."-Reuters