* Visiting Iranian official endorses Assad
* Fighters running low on ammunition
* Fighting follows defection of prime minister
By Hadeel Al Shalchi
ALEPPO, Syria, Aug 7 (Reuters) - Syria's President Bashar al-Assad won a pledge of support on Tuesday from regional ally Iran as his forces tried to choke off rebels in the northern city of Aleppo.
Seeking to restore his authority after suffering the gravest setbacks so far in the 17-month-old uprising, culminating in the defection of his prime minister on Monday, Assad was shown on television meeting a senior Iranian official.
It was the first footage broadcast of the 46-year-old leader for two weeks, and came a day after Syria's new caretaker prime minister was televised chairing a hastily called cabinet session, possibly to rebut reports that other ministers had deserted along with premier Riyad Hijab.
Saeed Jalili, head of Iran's Supreme National Security Council, said Iran would not let its close partnership with the Syrian leadership to be shaken by the uprising or external foes.
"Iran will not allow the axis of resistance, of which it considers Syria to be an essential part, to be broken in any way," Syrian television quoted Jalili as saying.
The "axis of resistance" refers to Shi'ite Iran's anti-Israel alliance with Syria's rulers - from the Alawite faith which is an offshoot of Shi'ite Islam - and the Lebanese Shi'ite militant group Hezbollah, which fought a month-long war with Israel in 2006, with Iranian and Syrian support.
Damascus and Tehran have held Sunni Muslim Gulf Arab states and Turkey, all allies of the United States and European powers, responsible for the bloodshed in Syria by supporting the overwhelmingly Sunni Muslim rebels. Western powers sympathetic to the rebels are concerned that anti-Western Sunni Islamists could benefit from a victory for the anti-Assad forces.
Iran's Fars news agency said Jalili told Assad that Iran was prepared to provide humanitarian aid to Syria.
On a fence-mending visit to Turkey, Iranian Foreign Minister Ali Akbar Salehi said he wanted to work with Ankara to resolve the crisis. Turkish Prime Minister Tayyip Erdogan described as "worrying" a comment on Monday by Tehran's top general, who blamed Turkey, Saudi Arabia and Qatar for bloodshed in Syria.
Iran has expressed fears for more than 40 Iranians it says are religious pilgrims kidnapped by rebels from a bus in Damascus while visiting Shi'ite shrines. Salehi wrote to U.N. Secretary-General Ban Ki-moon seeking his help to free them.
Rebels say they suspect the captives were troops sent to help Assad. A rebel spokesman in the Damascus area said on Monday three of the Iranians had been killed by government shelling. He initially said the rest would be executed if the shelling did not stop but later said they were being questioned.
U.S. Secretary of State Hillary Clinton, without naming Iran or Sunni powers, warned against a descent into "sectarian warfare" and said Washington would not tolerate "sending in proxies or terrorist fighters" to "exploit" Syria's conflict.
In Aleppo, rebels trying to fight off an army offensive said they were running low on ammunition as Assad's forces tried to encircle their stronghold in the southern approaches to the country's biggest city.
Assad has reinforced his troops in preparation for an assault to recapture rebel-held districts of Aleppo after repelling fighters from most of Damascus.
"The Syrian army is trying to encircle us from two sides of Salaheddine," said Sheikh Tawfiq, one of the rebel commanders, referring to the southwestern Aleppo neighbourhood which has seen heavy fighting over the last week.
Mortar fire and tank shells exploded across the district early on Tuesday, forcing rebel fighters to take cover in crumbling buildings and rubble-strewn alleyways.
Tanks have entered parts of Salaheddine and army snipers, using the cover of heavy bombardment, deployed on rooftops, hindering rebel movements.
Another rebel commander, Abu Ali, said snipers at the main Saleheddine traffic roundabout were preventing the rebels from bringing in reinforcements and supplies. He said five of his fighters were killed on Monday and 20 wounded.
But rebels said they were still holding the main streets of Salaheddine.
A fighter jet pounded targets in the eastern districts of Aleppo and shelling could be heard in the early morning, an activist in Aleppo said.
"Two families, about 14 people in total, were believed killed when a shell hit their home and it collapsed this morning," the activist said. The house was one street away from a school being used as a base by rebels, he said.
As Assad's forces battle to retake Aleppo, fighting has continued across the country. The Syrian Observatory for Human Rights, which monitors the violence, said more than 270 people - including 62 soldiers - were killed in Syria on Monday, one of the highest death tolls in an uprising in which activists reckon at least 18,000 have died.
Sixty-four of those killed on Monday died in the city of Aleppo and its surrounding province, the Observatory said.
The president has suffered a series of blows in the last three weeks, from the bombing of his inner circle to the rebel gains in Aleppo, at border crossings and briefly in Damascus.
On Monday, Hijab denounced Assad's "terrorist regime" after fleeing the country.
The defection of Hijab, who like most of the opposition hails from the Sunni Muslim majority, was a further sign of the isolation of Assad's government around an inner core of powerful members of his minority Alawite sect.
Opposition figures, buoyant despite setbacks in recent weeks of fighting, spoke of an extensive and long-planned operation to spirit Hijab and his extended family over the Jordanian border.
A spokesman for U.S. President Barack Obama hailed Hijab's defection as a sign that the 40-year rule of Assad's family was "crumbling from within" and said he should step down.
Western leaders' repeated predictions of Assad's imminent collapse have so far proven premature, however.
The security forces have overwhelming superiority in firepower, which they have wielded against lightly armed rebels.
Hijab's defection was the latest sign of Sunnis abandoning Assad, but there has been no sign yet that members of his mainly Alawite ruling inner circle are losing their will to fight on.