The Islamic State in Iraq and the Levant said it would target "the main joints of the regime in imprisoned Damascus," after meetings with eight Syrian factions.
"Targets include security branches, support and supply points, training centres, and infrastructure," the message said, cited by the US-based terrorist-tracking website SITE Intelligence.
The United States and its allies are planning a military strike against Syria following the 21 August attack on several Damascus suburbs, in which over 300 people died and thousands were injured.
British prime minister David Cameron announced via Twitter on Wednesday that the UK will put a resolution to the United Nation Security Council later in the day "authorising necessary measures to protect civilians" in Syria.
The Syrian government has strongly denied that it used chemical weapons and claims it has proof that implicates rebel fighters for the attack.
A team of UN weapons inspectors is probing the 21 August attack which may be the worst chemical weapons attack since Iraqi leader Saddam Hussein gassed thousands of Kurds in 1988.
Russia, China and Iran have separately warned that a military intervention in Syria would be "disastrous" for the region.