"The US threats and possible intervention in Syria is a disaster for the region and if such an act is done, certainly, the Americans will sustain damage like when they interfered in Iraq and Afghanistan," Ayatollah Khamenei said in a meeting with members of the new Iranian cabinet in Tehran on Wednesday.
He cautioned the US and its allies that their military intervention in Syria would yield no result but blazing fire and increasing nations' hatred from them.
Ayatollah Khamenei warned the US and its allies that "starting this fire will be like a spark in a large store of gunpowder, with unclear and unspecified outcomes and consequences".
On Saturday, US Defense Secretary Chuck Hagel claimed that the White House is studying different military options against Syria.
Hagel suggested the Pentagon is moving forces into place ahead of possible military action against Syria, even as President Barack Obama voiced caution.
Syrian Foreign Minister Walid al-Mualem said on Tuesday that Syria will press on with its military efforts despite any potential foreign strikes on its territory, adding that any strike would serve the interests of al Qaeda-linked rebel groups.
"The war effort led by the United States and their allies will serve the interests of Israel and secondly the Al-Nusra Front," Mualem told a press conference in Damascus.
"The (government's) military effort will not stop around Damascus. If the purpose is to limit the victories of our armed forces, they will not be successful," Mualem said.
Any foreign strike on Syria to try to create a balance of power in the war between President Bashar al-Assad's forces and the rebels is "delusional", the foreign minister said.
"If the purpose of a possible (foreign) military strike is to achieve a balance of power ... it's delusional and not at all possible," Mualem added.
"If they wanted to attack Syria using claims - utterly incorrect claims - about the use of chemical weapons, I defy them to provide what evidence they have," he said.
Russia warned on Tuesday that a military intervention in Syria could have "catastrophic consequences" for the region, and called on the international community to show "prudence" over the crisis.
"Attempts to bypass the Security Council, once again to create artificial groundless excuses for a military intervention in the region are fraught with new suffering in Syria and catastrophic consequences for other countries of the Middle East and North Africa," a foreign ministry spokesman said.
"We are calling on our American partners and all members of the world community to demonstrate prudence (and) strict observance of international law, especially the fundamental principles of the UN Charter," ministry spokesman Alexander Lukashevich said in a statement.