Iran pledged support for Syria’s beleaguered government Tuesday as forces loyal to President Bashar al-Assad battled rebels for control of Aleppo, unleashing intense bombardments from the air and ground that forced thousands of civilian inhabitants to flee the country’s largest city.

In a high-level show of support, Saeed Jalili, the head of Iran’s Supreme National Security Council, met with Assad in Damascus and vowed that Iran would help its ally confront “attempts at blatant foreign interference” in Syria’s internal affairs, the official Syrian Arab News Agency (SANA) reported.

Video footage of the meeting, broadcast on Syrian state television, gave Syrians their first glimpse of Assad in nearly three weeks — since he was shown on TV swearing in a new defense minister to replace one of four top security officials who were assassinated in a July 18 bombing.

“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 during the meeting. The Iranian, who is also his country’s lead nuclear negotiator, was referring to Iran’s alliance with Syria and the Lebanese Shiite militant group Hezbollah in confronting Israel.

In the northern city of Aleppo, Syria’s commercial capital, fighting raged between government forces and rebels near the city center Tuesday, opposition groups reported. They said Assad’s forces battered neighborhoods across the city with aerial bombing, artillery shells and rockets fired from helicopters.

“I heard a huge explosion this afternoon and went to the roof to see,” said a 28-year-old Aleppo resident who asked that his name not be used for his safety. “I was told by a friend who is closer to that area that two buildings in that street were shelled and they fell down.”

The Syrian army also shelled Damascus and a number of smaller cities Tuesday. At least 140 people were killed across the country, including 31 dead in Damascus, 29 in Homs and 27 in Aleppo, according to the Local Coordination Committees, an activist network.

Rebel fighters in Aleppo said they were running low on ammunition as they sought to hold neighborhoods on the southern edge of the city, Reuters news agency reported. Assad’s forces blasted rebel positions with tanks and mortars and deployed snipers on rooftops. Meanwhile, a fighter jet carried out airstrikes on the eastern districts of Aleppo.

The assault has forced thousands of frightened refugees to flee the city for safety, news agencies reported.

In South Africa, visiting Secretary of State Hillary Rodham Clinton said Tuesday that the fighting and a series of defections from Assad’s government underscore the urgency of planning for his departure.

“The intensity of the fighting in Aleppo, the defections, really point out how imperative it is that we come together and work toward a good transition plan,” Clinton told reporters at a news conference with South Africa’s foreign minister. She is scheduled to travel to Istanbul, Turkey, this weekend for talks with Turkish officials and Syrian opposition figures.

Clinton also issued a veiled warning against Iranian involvement in a Syrian conflict that she suggested could turn into sectarian warfare. “Those who are attempting to exploit the misery of the Syrian people, either by sending in proxies or sending in terrorist fighters, must recognize that that will not be tolerated first and foremost by the Syrian people,” she said.

During his meeting with Jalili, Assad “stressed the Syrian people and government’s determination to clear the country of the terrorists and fight terrorism without tolerance,” SANA reported.

Jalili was quoted as saying that the only way to resolve the unrest in the country would be to find a “Syrian solution.”

Jalili’s visit to Damascus was also aimed in part at seeking ways to secure the release of 48 Iranians captured by rebels just outside the capital on Saturday.

The Iranian government claims that its captured nationals were Shiite pilgrims on their way to Sayida Zeinab, a Muslim shrine south of Damascus that is popular with Shiites. But rebels assert that the Iranians belong to their country’s elite Revolutionary Guard Corps and were on a mission to help the Assad government battle Syria’s persistent 17-month-long uprising.

“Kidnapping innocent people is not acceptable anywhere in the world,” Jalili said, according to Iran’s official Islamic Republic News Agency. He said Iran would do what it could to “secure release of the 48 innocent pilgrims kidnapped in Syria.”

Iranian Foreign Minister Ali Akbar Salehi also traveled to Turkey on Tuesday in an additional diplomatic push to free the captives. The government of Turkey is close to the Syrian opposition.

The fate of the Iranians remained clear Tuesday. Three members of the group were reportedly killed by government shelling on Monday, according to rebels from the Free Syrian Army.

In a letter Tuesday to U.N. Secretary General Ban Ki-moon, Salehi referred to “media reports” about the three deaths and said that “the hostage takers have threatened to kill the remaining captives in the coming hours.” He said the Iranians were abducted while “on their way to the Damascus Airport.” The letter requested Ban’s help in securing the immediate release of the Iranians.

Late on Monday, the Iranian Foreign Ministry summoned the Swiss charge d’affaires in Tehran to protest the kidnapping and emphasize that the U.S. government would be held responsible for the fate of the Iranians. The Swiss Embassy in Tehran has represented U.S. interests in the country since Iran and the United States officially severed diplomatic ties in 1980.

Before going to Damascus, Jalili stopped in Lebanon on Monday, stirring the ire of Lebanese political factions opposed to Assad. “The visit is against the interest of the Lebanese people who stand by their brothers in Syria,” former prime minister Saad Hariri said in a statement. He added, “Iran should know that all these threats and suspicious moves will not delay the fall of its ally Bashar al-Assad.”

Jalili’s visit came a day after Syria’s prime minister defected to Jordan, becoming the most senior official to quit Assad’s embattled government, according to rebels who claim they helped him escape.

The reported defection of Prime Minister Riyad Hijab buoyed the rebels, who saw it as a clear sign that top officials are abandoning Assad as he attempts

A statement attributed to Hijab and read on the al-Jazeera Arabic news channel Monday said he had resigned to protest his government’s harsh tactics in confronting the opposition.

“I am announcing that I am defecting from this regime, which is a murderous and terrorist regime,” the statement said. “I join the ranks of this dignified revolution.”

Real power in Syria is wielded by Assad’s inner circle of friends, family and the powerful chiefs of his security forces. But the defection of the head of Assad’s government nonetheless sent a strong signal that his support is rapidly unraveling even within the ranks of those assumed to still be loyal.

Hijab, a former agriculture minister and a member of the ruling Baath Party, is a Sunni Muslim from the eastern town of Deir al-Zour, which has been in open revolt against the government for more than a year.

Reuters news service quoted an unidentified Jordanian government official as confirming that Hijab had defected and taken refuge there. Syrian state television, however, reported that Hijab had been fired, less than two months after he was appointed to the job. Deputy Prime Minister Omar Galawanji was appointed as the head of a caretaker government, according to the official Syrian Arab News Agency (SANA).

Hijab’s departure followed an accelerating stream of defections from Syria’s armed forces, including that of Brig. Gen. Manaf Tlas, a former confidant and close friend of Assad’s who fled to Turkey a month ago, then went to France to join his father, a once-powerful former defense minister.

A senior State Department official traveling with Secretary of State Hillary Rodham Clinton in South Africa said that the defection, if confirmed, would represent “further evidence that the Assad regime is crumbling.’’

“Its days are numbered, and we call on other senior members of the regime and the military to break with the bloody past and help chart a new path for Syria — one that is peaceful, democratic, inclusive and just,’’ the senior State Department official said.

The Syrian military blasted Damascus and at least half a dozen cities around the country Monday with artillery as fierce clashes rocked Aleppo. At least 116 people were killed across Syria on Monday, including 30 in Aleppo and 29 in Damascus and its suburbs, according to the Local Coordination Committees, an activist network.

In Damascus, a bomb exploded Monday in the state television offices, causing minor injuries, according to SANA. Photos taken after the blast, which hit the third floor of the building, showed a demolished roof with wires hanging down.

The complicated operation to get Hijab out of the country was completed in a series of carefully planned steps by the Free Syrian Army, according to Col. Malik Kurdi, a deputy commander with the rebel force.

“The prime minister and his family were transferred outside Syria to Jordan by separate vehicles and at different times,” Kurdi said. “The defectors cannot leave in an hour or a day. The process takes a long time, and there are many phases and routes.”

Jordanian authorities may not have initially known about Hijab’s entry into the country because he was brought via smuggling routes, Kurdi said. But Jordanian contacts eventually met him once he crossed the border. Kurdi predicted that the successful escape would lead to more defections among other top officials who have been thinking of leaving the country.

Sly reported from Antakya, Turkey. Anne Gearan in South Africa, William Branigin and Greg Miller in Washington, and Suzan Haidamous and Ahmed Ramadan in Beirut contributed to this report.