President Bashar al-Assad vowed on Tuesday to crush the 17-month rebellion against his regime and to cleanse Syria of 'terrorists', as his troops engaged rebels in key battleground city Aleppo.

"The Syrian people and their government are determined to purge the country of terrorists and to fight the terrorists without respite," he was quoted by state news agency SANA as telling visiting senior Iranian envoy Saeed Jalili.

Assad had earlier appeared on television for the first time in more than two weeks in a meeting with Jalili, a top aide to Iranian supreme leader Ayatollah Ali Khamenei.

Jalili offered Assad his country's backing, saying Tehran would "never allow the resistance axis — of which Syria is an essential pillar — to break.

"What is happening in Syria is not an internal issue but a conflict between the axis of resistance on the one hand, and the regional and global enemies of this axis on the other," he added.

Iran has accused Turkey and Gulf Arab countries of arming the Syrian opposition in collusion with the United States and Israel, to overthrow the Assad regime.

Jalili was previously quoted as saying "the crisis in Syria must be solved internally, through national dialogue, and not through the intervention of external forces".

He added: "The Syrian people are hostile to any plan supported by the Zionists and the US".

Later, Jalili called for an end to "all foreign intervention" in Syria, adding that Tehran rejects "any party imposing its will through military intervention".

Assad said his country was "able to defeat foreign plans targeting the resistance axis and Syria's role in it".

Tehran also sent its foreign minister to Ankara and a letter to Washington holding them responsible for the fate of 48 kidnapped Iranians.

In Aleppo, clashes rocked several central areas of the city while the army also shelled rebel-held areas in the east, the Syrian Observatory for Human Rights said.

The fighting in and around Aleppo killed at least 20 people, the watchdog said, adding that the nationwide toll was 122.

Aleppo has been bracing for a major ground offensive after a senior security official said the army had completed a buildup of some 20,000 troops.

Near Homs in central Syria, opposition gunmen attacked an electricity company housing compound, killing 16 people, including Alawites, Christians and Sunnis, the Observatory said.

And rebels attacked an oil field in the eastern province of Deir Ezzor, triggering clashes in which four rebels and six soldiers were killed, it added.

Assad 'may retreat to Alawite area'

Defected ex-premier Riad Hijab was in neighbouring Jordan firming up his plans after his defection to the opposition, which Washington said showed the regime was crumbling.

In that vein, Jordan's King Abdullah II said Assad might make a "worst case scenario" retreat to an Alawite stronghold if he falls from power.

Some experts have also predicted that if Damascus falls to rebels, Assad could take refuge among Alawites in the northeastern mountains of Syria, where opposition forces say he has already been stockpiling weapons.

"I have a feeling that if he can't rule Greater Syria, then maybe an Alawi enclave is Plan B," Abdullah said in an interview with US television network CBS.

"That would be, I think for us, the worst case scenario — because that means then the breakup of Greater Syria.

"That means that everybody starts land grabbing which makes no sense to me. If Syria then implodes on itself that would create problems that would take decades for us to come back from."

King Abdullah predicted Assad would keep up his brutal crackdown to cling to power because he "believes that he is in the right.

"In his mentality, he is going to stick to his guns... I think the regime feels that it has no alternative, but to continue... I don't think it's just Bashar. It's not the individual. It's the system of the regime."

Meanwhile, following the Damascus talks, Jalili told Al-Alam that Tehran was using "all means possible" to secure the release of its 48 abducted citizens.

Tehran says they are Shiite pilgrims, while their rebel captors insist they are members of Iran's elite Revolutionary Guards.

Iranian Foreign Minister Ali Akbar Salehi headed to Turkey to demand Ankara's assistance in securing the release of the Iranian hostages amid growing concern for their fate.

Salehi later said "Turkey has links with the opposition in Syria, so we think Turkey can play a major role in freeing our pilgrims".

Earlier, Iran said that, "considering that the (rebel) Free Syrian Army -- which claims to have abducted the Iranian pilgrims -- is backed by Turkey, the visit by the foreign minister aims to warn and remind the Ankara government of its responsibilities in this matter".

Tehran delivered a similar message to Washington in a letter transmitted through the US interests section of the Swiss embassy.

"Because of the United States' manifest support of terrorist groups and the dispatch of weapons to Syria, the United States is responsible for the lives of the 48 Iranian pilgrims abducted in Damascus," Deputy foreign minister Hossein Amir Abdollahian quoted the letter as saying.

A letter to UN chief Ban Ki-moon highlighted reported comments by Syrian rebels that three of the Iranians had been killed in government shelling.

"I would like to seek the cooperation and the good offices of your excellency for securing the release of these hostages," Salehi wrote.

"The kind cooperation of the relevant United Nations offices in responding to this request of government and the families of the hostages will be highly appreciated," he added.

In other developments, outgoing UN-Arab League envoy Kofi Annan and Lebanon said they would not send a representative to a meeting on the Syria conflict to be hosted by Iran on Thursday.

Iran had called on Monday for a ministerial meeting among states that have taken a "realistic and principled stand" on the Syrian crisis.

Diplomatic sources at the UN meanwhile said Tuesday the UN Security Council would hold a ministerial meeting on the conflict on August 30.

The meeting has been called by France as president of the Security Council for August, sources from several countries said, speaking on condition of anonymity because no official announcement has yet been made.

Diplomats, speaking on condition of anonymity, said it was not yet sure that Russia and China, which have vetoed three council resolutions on Syria, would attend at ministerial level.

And on the humanitarian front, more than 22,000 Iraqis have fled Syria in less than three weeks, while 12,600 Syrians have done so since the beginning of the year, the UNHCR said.

In Geneva, the World Health Organisation said Syrians urgently needed life-saving medicines, and the World Food Programme said 1.5 million people in rural areas would need food aid in the next three to six months.

And Britain announced a grant of £10 million ($15.6 million, 12.6 million euros) to aid thousands of Syrian refugees who have fled to Jordan, Lebanon, Turkey and Iraq.