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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. secretary of state urged stepped up international planning for the regime's collapse.

The contrasts couldn't have been more vivid: Assad and Iran's Saeed Jalili vowing to defeat the rebels and their backers, while Secretary of State Hillary Rodham Clinton predicted Assad's regime was quickly unraveling, with high-level defections such as his prime minister's switch to the rebel side.

It also 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 by Jalili 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. They include the Sunni-led rebels and Assad's minority Alawite community, an offshoot of Shiite Islam and part of its close bonds with Shiite power Iran.

A growing humanitarian crisis is already taking hold.

More than 1,300 Syrians fled to Turkey on Tuesday 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 U.N. officials in Baghdad as they prepared for more refugees.

In Geneva, meanwhile, the World Health Organization said the fighting has severely hit Syria's health services, including closing down 90 percent 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 Tuesday near the historic city center. That suggested the rebels were making some inroads in Aleppo, which lies some 25 miles 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 1,328 Syrian refugees had crossed the border Tuesday -- 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 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 Tuesday 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.

Despite a ferocious crackdown, rebels in Syria have grown more confident and are using increasingly bolder tactics both in Aleppo and in the capital, Damascus.

In a daylight attack, rebels on Saturday abducted a group of 48 Iranians near Damascus, branding them as spies assisting in Assad's crackdown.

Iran says those captured when their bus was commandeered were pilgrims visiting a Shiite shrine on the outskirts of Damascus, and Jalili said Tuesday that Iran would spare no effort to secure their release.

"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.

Meanwhile, Iran's Foreign Ministry said it holds the U.S. responsible for the fate of the abducted Iranians.

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

The Iranian Embassy in Turkey said Iran's Foreign Minister Ali Akbar Salehi was traveling Tuesday to Turkey, where he was to meet with his Turkish counterpart to discuss Syria and the abducted Iranians.

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 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.

The representative's claim that three were killed could not be independently verified. An official at the Iranian Embassy in Damascus said he had no information on the subject.