Syrian President Bashar Assad made his first appearance on state TV in nearly three weeks today in a show of solidarity with a senior Iranian envoy, even as the US urged stepped up international planning for the regime's collapse.

The visit to Damascus by the highest-ranking Iranian official since the uprising began co-incided with a warning by an increasingly agitated Tehran that it holds the US 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 US Secretary of State Hillary Rodham Clinton predicted Assad's regime was quickly unravelling, 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.

"I am not going to put a timeline on it. I can't possibly predict it, but I know it's going to happen as do most observers around the world," Clinton told reporters.

A post-Assad Syria presents a host of worrisome scenarios, including a bloody cycle of revenge and power grabs by the country's patchwork of factions, including the Sunni-led rebels and Assad's minority Alawites, an offshoot of Shiite Islam allied with Shiite Iran.

Despite a ferocious government crackdown, the Syrian rebels have grown more confident and are using increasingly bolder tactics. They seized the 48 Iranians in a bold daylight attack near Damascus at the weekend, claiming they were members of Iran's powerful Revolutionary Guards sent on a "reconnaissance mission" to assist in Assad's crackdown.

Iran says the Iranians, who were captured when their bus was commandeered near the airport, were pilgrims visiting a Shiite shrine on the outskirts of Damascus.

Iran has issued a flurry of appeals for their release, with Jalili saying that Iran would spare no effort to secure their freedom.

"We believe that the abduction of innocent people could not be accepted by any rational person. We believe that the parties that support those terrorist groups to commit such disgraceful acts, are their partners," he said.

In a pointed message to the Obama administration, Iran's Foreign Ministry said that it holds the US responsible for the fate of the abducted Iranians.

Iran's state IRNA news agency said the ministry summoned the Swiss envoy in Tehran yesterday to stress that Iran expects Washington to use its influence to secure the Iranians' release. The Swiss look after US interests in Iran since Tehran and Washington have no diplomatic relations.

Meanwhile, Iran's Foreign Minister Ali Akbar Salehi was travelling to Turkey, where he was to meet with his Turkish counterpart to discuss Syria and the abducted Iranians. Salehi also sent an appeal to UN Secretary General Ban Ki-moon for help in securing the Iranians' release.

Syrian rebels claimed three of the Iranian captives were killed on Monday during shelling by government forces in Damascus and its suburbs, and threatened to kill the remaining Iranians unless the army stopped its bombardment. The rebel claim could not be independently verified.

"The Syrian regime is responsible for anything that happens to the Iranians," a representative of the Baraa Brigades, which claimed responsibility for the group's abduction, told The Associated Press on Skype.

Meanwhile, a growing humanitarian crisis was already taking hold in Syria.

More than 1300 Syrians fled to Turkey yesterday as rebels tried to expand their hold inside Aleppo, Syria's largest city, despite two weeks of withering counterattacks by Assad's troops. Close to 48,000 Syrians have already taken refuge in Turkey, which has served as a staging ground for rebels. Even more refugees have crossed into Jordan and Lebanon.

And at least 22,300 Iraqis who fled to Syria several years ago have streamed home in the past three weeks, said UN officials in Baghdad as they prepared for more refugees.

In Geneva, meanwhile, the World Health Organisation said the fighting has severely hit Syria's health services, including closing down 90 per cent of pharmaceutical plants in Damascus and other main cities and leaving critical shortages of medicine. WHO spokesman Tarik Jasarevic cited a Syrian Health Ministry report that 200 ambulances were lost in recent weeks to theft or clashes.

Aleppo-based activists said clashes were going on today near the historic city centre. That suggested the rebels were making some inroads in Aleppo, which lies some 25 miles (40 kilometres) from the Turkish border.

Intense government bombardment of the Syrian town of Tal Rafaat closer to the border sent scores of people spilling into Turkey for safety, according to the activists.

A Turkish government official said 1328 Syrian refugees had crossed the border yesterday - nearly double the number of the previous day. The official spoke on condition of anonymity in line with government rules.

Ahmad Saleh, a Tal Rifaat resident who fled to Turkey, said the town was shelled on Monday from the nearby air base of Minnegh, killing at least two people. "We had to choose between dying in Syria and coming to Turkey," he said.

A video posted online by activists showed a large group of Free Syrian Army rebels in military fatigues and carrying rocket-propelled grenades and automatic rifles. The fighters were seen announcing that they were joining the "Unification Brigade," the main group of rebels in Aleppo, to assist in the "liberation" of the city.

"They have Satan on their side, we have God on ours," one rebel shouted. "We are coming, Aleppo," shouted another. The authenticity of the video could not be independently verified.

- AP