Iran warned on Tuesday against foreign military intervention in Syria, saying the resulting conflict would engulf the region.

The United States said on Monday it believed Syrian President Bashar Assad was responsible for a chemical weapons attack against civilians last week, and U.S Secretary of State John Kerry said President Barack Obama was consulting with allies before deciding how to respond.

U.S. warships armed with cruise missiles are already positioned in the Mediterranean.

Iran, which is supporting Assad against rebels seeking to overthrow him, has said rebels were behind the suspected attack and said on Tuesday the West was using it as a pretext to intervene in Syria.

"We want to strongly warn against any military attack in Syria. There will definitely be perilous consequences for the region," Iranian Foreign Ministry spokesman Abbas Araqchi told a news conference. "These complications and consequences will not be restricted to Syria. It will engulf the whole region."

Shi'ite Iran is Syria's closest ally and has accused an alliance of militant Sunni Islamists, Israel and Western powers of trying to use the conflict to take over the region.

As well as backing Assad, Iran also supports the Lebanese Shi'ite militant group Hezbollah which has sent fighters into Syria to help the government there. Tension is also rising between Hezbollah and Israel over cross-border rocket attacks.