By Parisa Hafezi
ANKARA (Reuters) - Iran's Supreme Leader Ayatollah Ali Khamenei said coordinated air strikes on Syria by the United States, France and Britain on Saturday were a crime that would bring no benefit.
"Today's dawn attack on Syria is a crime. I clearly declare that the president of the United States, the president of France and the British prime minister are criminals," Khamenei said in a speech, according to his Twitter account.
"They will not benefit (from the attack) as they went to Iraq, Syria and Afghanistan in the past years and committed such crimes and did not gain any benefits," Khamenei added.
Iran -- the dominant Shi'ite Muslim power which is in rivalry with Saudi Arabia and the United States' other Sunni Arab allies -- has been Syrian President Bashar al-Assad's most supportive ally against insurgents throughout the conflict.
Militias backed by Tehran helped Assad's army stem rebel advances and, following Russia's entry into the war in 2015, turn the tide decisively in the Syrian government's favor.
Iran's pragmatist President Hassan Rouhani warned that the U.S.-led missile attacks would lead to further destruction in the Middle East, the semi-official Tasnim news agency reported.
"Such attacks will have no result but more destruction ... the Americans want to justify their presence in the region by such attacks," Rouhani was quoted as saying, signaling that Iran's support for Assad would grow.
An official in Iran's Revolutionary Guards Corps (IRGC), the Islamic Republic's most powerful arm, said fallout from the attacks would harm Washington.
"With this attack ... the situation will become more complex, and this will surely be at the expense of the United States, which will be responsible for the aftermath of upcoming regional events that will certainly not be in their interest," Yadollah Javani, the Guards' deputy head for political affairs, told Fars news agency.
"The resistance front will be strengthened and it will have more capacity to act against (U.S.) acts of intervention. Americans should expect the consequences of their actions," Javani said.
Iran often refers to regional countries and forces opposed to Israel and the United States as a "resistance front".
In a statement, the IRGC said: "This unrelenting confrontation shows that the Syrian people, with the support of the strategic allies of Damascus, will not stop until achieving complete victory," Fars reported.
Iran's military chief of staff General Mohammad Baqeri assured Syrian Defense Minister Ali Abdullah Ayoub of Tehran's continued support.
"The nation and the armed forces of the Islamic Republic of Iran will continue to fight alongside the Syrian people and the Syrian armed forces against the criminal terrorists," Fars quoted Baqeri as telling Ayoub by telephone.
Earlier, the Foreign Ministry in Tehran said Washington and its allies had attacked Syria "despite the absence of any proven evidence."
"Iran is opposed to the use of chemical weapons on the basis of religious, legal and ethical standards, while at the same time it ... strongly condemns (using this) as an excuse to commit aggression against a sovereign state," it said in a statement carried by state media.
Analyst Hossein Sheikholeslam, a former Iranian ambassador to Damascus, told state television the attacks would bolster support for the Syrian government.
"These attacks will stabilize the Syrian government... and unite the different tribes in Syria as Syrians become aware of their honor and come to the defense of the independence, territorial integrity and the government of their country," he said.
(Reporting by Parisa Hafezi and Dubai newsroom; Writing by Samia Nakhoul and Parisa Hafezi; Editing by Jason Neely and Helen Popper)