The Syrian army has launched a ground assault on the northern city of Aleppo, sparking fierce clashes with opposition fighters in the frontline district of Salaheddine.

“The army is advancing from west to east to cut Salaheddin in half horizontally,” an official said on Wednesday on condition of anonymity, referring to the key rebel stronghold in the city.

Wassel Ayub, a commander in the rebel Free Syrian Army, said: “The regime forces advanced into Al-Malaab Street with tanks and armoured vehicles and fierce fighting is now taking place in the area.”

An Al Jazeera correspondent said “a large number of people have been killed or injured in fierce battle near Salaheddine in which advanced Russian tanks have been used by the government forces”.

Clashes have also been reported in Hanano, Tareeq Al Bab and Sha’ar in the besieged city, the UK-based Syrian Observatory for Human Rights (SOHR) said.

The observatory said that the clashes taking place in the streets of Salaheddin and in surrounding areas were the most fierce that the northern city has seen in the nearly 17-month uprising.

SOHR said neighbourhoods of Maysaloun, Sakhour and Tal Rifaat were under shelling by government forces.

The Syrian army has made progress but rebels have not abandoned Salaheddine, the Al Jazeera correspondent said, adding that the FSA has shot down a plane and destroyed five tanks in Aleppo.

The army, which has been massing its troops and armour in and around Aleppo since late last month, was moving from west to east, coming from Hamdaniyeh, a district adjacent to Salaheddine, the FSA’s Ayub said.

Al Jazeera’s Rula Amin, reporting from neighbouring Beirut, said: “It’s not just symbolic but also where most of Free Syrian Army is concentrated, and for the world it also became symbol of FSA’s major success in getting their hold on this city.”

“FSA has been bringing in its own rebels from outside Aleppo from country side e.g. Idlib, Homs because for them it’s a major battle. Salahedinne is also crucial for government, as it has been a pillar of support for the president Assad in the last 16 months,” she said.

Retired guards among Iran hostages

In other developments, Iranian Foreign Minister Ali Akbar Salehi said on Wednesday that “retired” members of Iran’s Revolutionary Guards and army were among 48 Iranians taken hostage in Syria by rebels,” the ISNA news agency reported.

Salehi said the former military personnel were exclusively on a religious pilgrimage to Damascus when they were seized on Saturday.

- Anita McNaught reports from outside Aleppo

“A number of the [hostages] are retired members of the Guards and the army. Some others were from other ministries,” Salehi was quoted as telling reporters as he flew back from Turkey, which he asked for help in freeing the Iranians.

Another senior Iranian official visited Damascus on Tuesday where he met Syrian President Bashar al-Assad.

Saeed Jalili, a senior aide to Iran’s supreme leader Ayatollah Ali Khamenei, told Assad that Iran will continue to back the Syrian government.

During talks with Assad, Jalili said that what was happening in Syria was “not an internal issue”.

It is “a conflict between the axis of resistance on one hand, and the regional and global enemies of this axis on the other,” Jalili said.

On Monday, while on a visit to Beirut, the Lebanese capital, Jalili issued a veiled warning to countries backing the rebels.

“Those who believe that, by developing insecurity in the countries of the region by sending arms and exporting terrorism, they are buying security for themselves are wrong,” Iran’s official IRNA news agency quoted him as telling Adnan Mansour, Lebanon’s foreign minister.