Kerry said recent military gains by Assad's forces were only temporary and that if the Syrian leader believed that the counter-offensives against the rebels would be decisive, "then he is miscalculating".
Kerry was speaking in Amman before a meeting of the Friends of Syria group of nations - mainly Western and Arab opponents of Assad - trying to pave the way for U.S.-Russian proposed peace conference to end Syria's two-year conflict in which more than 80,000 people have been killed.
The conference aims to build on last year's international accord in Geneva which set out plans for a transition of power in Syria, but broke down over disagreements about Assad's role.
"In the event that we can't find that way forward, in the event that the Assad regime is unwilling to negotiate Geneva 1 in good faith, we will also talk about our continued support and growing support for the opposition in order to permit them to continue to be able to fight for the freedom of their country," Kerry told a news conference.
He said several thousand fighters from the Lebanese group Hezbollah were taking part in the Syrian conflict with active Iranian support on the ground.
"Just last week, obviously, Hezbollah intervened very, very significantly," he said "There are several thousands of Hezbollah militia forces on the ground in Syria who are contributing to this violence and we condemn that."
"...Active military support to the Assad regime simply exacerbates the sectarian tensions and it perpetuates ... the regime's campaign of terror against its own people."
Activists say 30 Hezbollah fighters were killed on Sunday in heavy fighting for the town of Qusair where the Lebanese Shi'ite guerrilla group joined Syrian troops trying to evict rebel fighters, who include radical Islamists and foreign jihadis.
The fighting for Qusair, which straddles strategic supply routes important for both Assad and the rebels, is the latest assault by Assad's forces seeking to consolidate control over territory linking the capital Damascus to his Alawite heartland overlooking the Mediterranean.
Although northern and eastern provinces remain beyond his grasp, the president has reasserted his military presence in the south and centre of the country, possibly seeking to send a delegation to peace talks from a position of greater strength.
Kerry said that without serious negotiations, Syria faced even deeper bloodshed.
"Our understanding (is that) if Geneva 2 were not on the horizon, all we would be looking at is the continued tragic disintegration of the county that will go down further into more violence and more destruction," he said.
"We believe that trying to get Geneva process - difficult as it, fraught with all the complications it represents - is a better alternative to the other."
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