UN chief Ban Ki-moon said on Thursday that President Bashar al-Assad should be "brought to justice" if his Damascus regime uses chemical weapons to combat the armed revolt in Syria.
"I have expressed my gravest concerns to (the) government of Syria and I have sent a letter directed to President Assad a couple of days ago," Ban told a joint news conference with Iraqi Prime Minister Nuri al-Maliki in Baghdad.
"I have warned that in any case, if chemical weapons is used, then whoever (it) may be will have to be brought to justice, and it will create serious consequences to those people," the UN secretary general said.
A top US defence official meanwhile told reporters in Baghdad that the use of chemical weapons in Syria was a "red line" for Washington.
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"The United States made clear that we are concerned about recent activity by the Syrian regime and that we would see any employment of chemical weapons as a red line," Under Secretary of Defence James Miller said.
"We also wish to emphasise the importance that... the Bashar al-Assad regime control and not employ its chemical weapons," he said.
The Syrian government, fighting to prevent the capital Damascus from falling to rebel forces, has insisted it would never resort to chemical weapons.
But Washington fears battlefield advances by rebels could prompt Assad to use chemical arms, or that such stocks could become insecure or find their way into the hands of groups hostile to the United States and its allies.
In a new warning as the conflict approaches the 21-month mark with more than 41,000 people killed, US President Barack Obama on Monday warned Assad of "consequences" if he uses chemical weapons against his own people.