BEIRUT — Syrian President Bashar Assad made his first appearance on state TV in nearly three weeks Tuesday in a show of solidarity with a senior Iranian envoy, even as the U.S. urged stepped-up international planning for the regime’s collapse.

The visit to Damascus by the highest-ranking Iranian official since the uprising began coincided with a warning by an increasingly agitated Tehran that it holds the U.S. responsible for the fate of 48 Iranians seized by Syrian rebels.

Appearing together on state TV, Assad and Iran’s Saeed Jalili vowed to defeat the rebels and their backers, while U.S. Secretary of State Hillary Clinton predicted Assad’s regime was quickly unraveling, with high-level defections such as his prime minister’s switch to the rebel side.

Jalili’s visit highlighted Assad’s deepening reliance on a shrinking list of allies, led by Tehran. Assad – seen on state TV for the first time since a July 18 bombing in Damascus killed four of his top security officials –- used Jalili’s visit to portray a sense of command and vowed to fight his opponents “relentlessly.”

Jalili, the secretary of Iran’s Supreme National Security Council, promised Iran would stand by Syria against its international “enemies” – a clear reference to the rebels’ Western backers and others such as Saudi Arabia and Qatar.

While there were no public pledges of greater military assistance to Assad, the mission appeared to reflect Iran’s efforts to reassure Syria of its backing and ease speculation that Tehran also could be making contingencies for Assad’s possible fall.

On a visit to South Africa, Clinton described Assad’s regime as splintering from Monday’s defection of Syria’s prime minister, Riad Hijab, and other military and political figures breaking away in recent months. She urged international leaders to begin work on a “good transition plan” to try to keep Syria from collapsing into more chaos after Assad.

Meanwhile, 48,000 Syrians have taken refuge in Turkey and even more refugees have crossed into Jordan and Lebanon.