The United States said on Monday it believed Syrian President Bashar Assad was responsible for a chemical weapons attack against civilians last week, and US Secretary of State John Kerry said President Barack Obama was consulting with allies before deciding how to respond.
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US 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."
Shiite 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 Shiite 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.
China's official Xinhua news agency also expressed opposition to intervention in Syria, as it said on Tuesday that Western countries were rushing to conclusions about who may have used chemical weapons before UN inspectors had completed their investigation.
"Such rhetoric, as well as the recent flurry of consultations between Washington and its allies, indicates that they have put the arrow on the bowstring and would shoot even without a UN mandate," Xinhua said in an English-language commentary.
"That would be irresponsible and dangerous. For starters, the current scenario is reminiscent of the lead-up to the Iraq War, which the United States staged with allegations about weapons of mass destruction that later turned out to be false."
Xinhua commentaries do not carry the same weight as government statements, but they can be read as a reflection of official Chinese thinking.
China has urged all parties not to jump to conclusions about the results of the UN probe, and has urged calmness in dealing with the accusations.
"In the heat of the crisis, all parties concerned should keep their heads cool, especially those impatient to take military actions without a UN mandate," it said.
"It is imperative that the Unites States and like-minded countries refrain from hasty armed invention and let the UN play its due part in determining how to act accordingly."
Russia, Assad's key ally and arms supplier, says rebel forces may have been behind the attack and has urged Washington not to use military force against Assad.
Moscow and Beijing have both vetoed previous Western efforts to impose UN penalties on Assad.
But China has been keen to show it is not taking sides and has urged the Syrian government to talk to the opposition and take steps to meet demands for political change. It has said a transitional government should be formed.
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