MUSCAT, Oman — As evidence grows of increased Iranian support for the Syrian government, the Obama administration’s strategy to bring the bitter fighting there to a close faces more challenges than ever.

With White House support, Secretary of State John Kerry has pushed for an international conference in Geneva in June that would bring representatives of the Syria government together with the opposition.

The aim would be to negotiate a transitional government that would take over if President Bashar al-Assad vacates his post and to put an end to the civil war that has killed more than 80,000.

But the stepped-up support Mr. Assad has received from Iran and Hezbollah in recent months appears to have fortified his belief that he can hang on to power and prevail militarily — or at least control a strategically significant swath of the country.