Allegations that the Syrian regime used chemical weapons last month are a "pretext" by the West to attack the country, Iran's Supreme Leader Ayatollah Ali Khamenei said.
Iran is Syria's main regional ally and has warned Western powers against intervening in the country's civil war, as the United States edges towards launching strikes against the Damascus regime.
Washington and its allies "are using the chemical weapon [allegation] as a pretext," and "are saying that they want to intervene for humanitarian reasons," Khamenei said on Thursday.
"The United States is wrong about Syria, and it is certain they will suffer... just like in Iraq and Afghanistan," Khamenei told members of the Assembly of Experts, the body that supervises his work.
Separately, the chief of Iran's elite Quds Force unit, Qassem Soleimani, said Tehran would back Syria "until the end" in the face of possible US-led military strikes.
Some analysts believe a wider goal of US President Barack Obama's determination to launch strikes is to blunt Tehran's growing regional influence and any consequent threat to Washington ally Israel.
"The aim of the United States is not to protect human rights... but to destroy the front of resistance [against Israel]," the Quds Force commander was quoted as saying by the media on Thursday.
"We will support Syria to the end," Soleimani added in his speech to the Assembly of Experts.
He did not elaborate on the nature of the support and Iran has constantly denied allegations by Western powers that it has sent military forces to prop up President Bashar al-Assad's embattled regime.
'Counsel and advice'
A year ago, the chief of Iran's Revolutionary Guards, Mohammad Ali Jafari, said members of the Quds Force foreign operations unit were in Syria to provide Assad's government with "counsel and advice".
Iran's Defence Minister Hossein Dehqan, meanwhile, ruled out sending troops or weapons to Syria.
"The Syrians do not need us to provide them with weapons because they have a defensive anti-aircraft system themselves," he was cited in the local media as saying.
President Hassan Rouhani said Iran will do "everything to prevent" an attack on the Syrian regime, according to extracts from statements published in the media.
"Any action against Syria is against the interests of the region but also against the friends of the United States in this region," he said.
"Such action will help nobody."
The US, France and other countries accuse Assad's forces of launching chemical weapons attacks on the outskirts of Damascus on August 21, which they say killed hundreds.
Obama is seeking congressional backing as well as broader international support for punitive strikes on Assad's regime.
Iran has previously warned that any military action against Syria risks sparking a broader regional conflagration.