Western governments are ready to increase support to opponents of President Bashar al-Assad if he rejects a political solution to Syria's civil war, U.S. Secretary of State John Kerry said on Wednesday.

Kerry said recent military gains by Assad's forces were only temporary and that if he believed the counter-offensives against the rebels would be decisive, "then he is mis-calculating".

Kerry, speaking at a news conference in Amman before a meeting of the Friends of Syria group of nations, said several thousand fighters from the Lebanese group Hezbollah were taking part in the conflict with active Iranian support on the ground.

"Just last week, obviously, Hezbollah intervened very, very significantly. There are several thousands of Hezbollah militia forces on the ground in Syria who are contributing to this violence and we condemn that."

He said U.S. President Barack Obama had made clear he did not intend to despatch U.S. forces to Syria but that Washington would discuss continued and growing support to help rebel fighters if Assad was unwilling to negotiate a political solution to the two-year-old crisis.

Wednesday's meeting in Amman, which gathers ministers from 11 Western and Middle Eastern countries who have led international opposition to Assad, aimed to promote efforts to end the bloodshed in Syria, he added.