Syrian TV has broadcast footage apparently showing President Bashar al-Assad meeting Iran’s security chief, the first time he has been seen on state television for several weeks. His appearance came a day after Prime Minister Riad Hijab defected to the opposition.
Arriving in Damascus, Iran’s security chief Saeed Jalili said only a "Syrian solution" would end the crisis.
Iran is President Assad’s staunchest regional ally.
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, Foreign Minister Ali Akbar Salehi is travelling to Turkey for talks in Ankara.
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, had travelled to Damascus from Lebanon.
He 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.
The rebels have claimed that the group are members of Iran’s Revolutionary Guard. Tehran says they are pilgrims who had been heading for a Shia religious site.