The Syrian opposition on Wednesday charged that regime forces had unleashed a chemical attack on civilians in Damascus overnight Tuesday. At least 20 people were asphyxiated from toxic gases, but there were no immediate fatalities, Al-Jazeera reported.

The rebels said the attack was carried out in Jubara, the northeastern Damascus suburb close to where an alleged chemical weapons strike last Wednesday killed hundreds.

One doctor in the area said he received six patients who had severe difficulty breathing and pain in their eyes, both symptoms typical of a chemical attack, Israel’s Walla News reported.

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Meanwhile, an explosion went off Wednesday morning near Syrian President Bashar Assad’s palace in the al-Muhajiroun area in Damascus. There were no immediate details about injuries.

Al-Jazeera also reported, citing rebel sources, that regime forces used white phosphorus or a substance it referred to as “napalm” in an overnight attack near Aleppo, leaving 12 dead and injuring at least 50.

The Arabic news site said the dead were university students. They showed severe burns and other skin wounds, according to the Syrian Media Center.

Last Wednesday, a massive alleged chemical attack in eastern Damascus killed some 355 people, according to the group Doctors Without Borders.

US Secretary of State John Kerry said Monday there was “undeniable” evidence of a large-scale chemical attack likely launched by Damascus. While US President Barack Obama hasn’t settled on how to respond to the attack, according to the White House, Defense Secretary Chuck Hagel said Tuesday that US forces are ready to act on any order by the president.

The Arab League also threw its weight behind calls for punitive action, blaming the Syrian government for the attack and calling for those responsible to be brought to justice.

British Prime Minister David Cameron on Tuesday recalled Parliament to hold an emergency vote Thursday on his country’s response. It is unlikely that any international military action would begin before then.

Russia, Syria and Iran have warned of grave consequences should action be taken.