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 countries of arming the opposition in Syria, in collusion with the United States and Israel, to overthrow the Assad regime.
Jalili was previously cited by Iran's Al-Alam television station 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."
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 Beirut on Monday, 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 saying.
In commercial capital Aleppo, clashes rocked several areas of the city centre on Tuesday, while the army also shelled rebel-held areas in the east, the Syrian Observatory for Human Rights said.
The fighting in Aleppo and its surrounding province killed at least 18 people, the watchdog said, adding the nationwide toll was 68.
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.
In other fighting on Tuesday, rebels attacked an oil field in the eastern province of Deir Ezzor, triggering clashes in which four rebels and six soldiers were killed, the Observatory said.
Defected ex-premier Riad Hijab was in neighbouring Jordan firming up his plans after his shock defection to the opposition, which Washington said showed Assad's regime was crumbling.
A US official called Hijab's defection "just the latest indication that Assad has lost control of Syria and that the momentum is with the opposition forces and the Syrian people."
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.
"I have a feeling that if he can't rule Greater Syria, then maybe an Alawi enclave is Plan B," he said.
"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 us decades for us to come back from."
King Abdullah predicted Assad would keep up his brutal security purge 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 citizens abducted by rebels.
Tehran says they are Shiite pilgrims, while the rebels claim 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.
That followed an unconfirmed report on a rebel group's Facebook page that three of them had been killed in shelling by regime forces on Monday.
"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," the Iranian foreign ministry said.
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.
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 representative in Baghdad said.
In Geneva, the World Health Organisation said Syrians urgently need life-saving medicines, and the World Food Programme said 1.5 million people in rural areas will 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.