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