Iran's security chief has told President Bashar al-Assad that Syria is part of a vital regional alliance that Tehran will not allow to be broken.
During talks in Damascus, Saeed Jalili said Syria was an essential part of an "axis of resistance".
The statement came a day after Syrian Prime Minister Riad Hijab defected to the opposition.
Syrian TV showed President Assad greeting Mr Jalili, his first TV appearance in two weeks.
President Assad was last seen on state TV on 22 July - four days after a bomb killed four security chiefs in Damascus - leading to speculation about his health and whereabouts.
State television said Mr Assad had told Mr Jalili of "the determination of the people and government of Syria to cleanse the country from the terrorists and fight against terrorism unreservedly".
He said Syria would "continue on the path of national dialogue" and that it was "capable of thwarting the foreign conspiracies".
Mr Jalili was quoted as saying: "Iran will not allow the axis of resistance, of which it considers Syria to be an essential part, to be broken in any way."
Correspondents say "axis of resistance" refers to Iran, Syria, Hezbollah in Lebanon and the Palestinian Islamist group Hamas in Gaza.
Tehran says it is planning an international meeting on Syria on Thursday.
It is also trying to secure the release of a group of Iranians abducted by rebels from a bus in Damascus on Saturday.
As well as Mr Jalili's visit to Damascus, Iranian Foreign Minister Ali Akbar Salehi is holding talks in the Turkish capital, Ankara.
'US held responsible'
An unconfirmed report from the rebels has suggested that three of the 48 hostages they are holding have been killed by army shelling.
Mr Jalili, who heads Iran's supreme national security council and is considered a senior aide to Supreme Leader Ayatollah Ali Khamenei, told reporters that "kidnapping innocent people is not acceptable anywhere", Iran's official news agency Irna reported.
Iran's Deputy Foreign Minister Hossein Amir Abdollahian has said it holds the US responsible for the hostages' safety.
He said the US was supporting "terrorist groups" and despatching weapons to Syria, and was therefore responsible for the lives of those abducted.
Rebels say the group are members of Iran's Revolutionary Guard. Tehran says they are pilgrims who had been heading for a Shia religious site.
'Business as usual'
The BBC's Jim Muir, in neighbouring Lebanon, said Syrian state media were giving the impression of it being business as usual in Damascus, despite Monday's defection.
Syria's information minister played down the significance of Mr Hijab's departure.
Syria was a state of institutions, Omran al-Zoubi said, and the flight of some of its individuals would not affect the state, however prominent they were.
But reports of defections have continued, with Turkey's foreign ministry announcing on Twitter that a general was among more than 1,300 refugees who fled across the border overnight.
The number of Syrians who have crossed into Turkey has risen to 47,500, Ankara says.
The UK said on Tuesday it was quadrupling its aid for refugees fleeing fighting in Syria.
Riad Hijab, appointed as prime minister less than two months ago, is the most prominent Syrian figure to defect so far.
Although his whereabouts are unclear, his spokesman appeared on al-Jazeera TV in neighbouring Jordan saying that the prime minister had fled Syria with his family and was in "a safe location".
US Secretary of State Hillary Clinton said on Tuesday that the international community had to plan for the fall of the Syrian regime.
Speaking in South Africa, she said: "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."
On the ground in Syria, clashes have been reported in Aleppo and the army is said to have stepped up its bombardment of the northern city.
Government forces are trying to dislodge rebel fighters who say they control up to half of Aleppo.
Opposition activists report intense attacks on rebel-held areas on the north-east and south-west sides of the city.
State media said troops had clashed with "terrorists" in several places inflicting heavy losses.
Further deaths were reported in Damascus on Tuesday. State news agency Sana said a "terrorist hideout" had been stormed by security forces.
British-based activist group Syrian Observatory for Human Rights said 270 people were killed across Syria on Monday. It said 61 civilians died in Aleppo province alone.
Activists estimate more than 20,000 people have died since the uprising against President Assad began in March last year.
Reports of casualties often cannot be independently verified.