How deadly was the attack?
A chemical weapons watchdog plans to inspect the rebel-held Syrian city where the Assad regime is accused of attacking its own people.
Families were reportedly found suffocated in their homes and shelters, with foam on their mouths.
The Syrian-American Medical Society said more than 500 people were brought to medical centres with symptoms "indicative of exposure to a chemical agent".
What has worldwide reaction been?
Last year, the USA carried out airstrikes against an airfield outside Homs after the Syrian regime was found to use chemical weapons.
The UN Secretary General has called for global investigators to have unfettered access after the alleged chemical attack in Syria.
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Pence, who the White House said will also travel to Brazil next month, was last summer said to have been far more conciliatory in his response to Maduro than Trump, who had threatened military intervention in Venezuela.
Trump's new national security adviser, John Bolton, advocated that Trump stay in Washington in the aftermath of the attack, said one of the officials.
The American president also criticized Russian Federation for its support of Syrian leader Bashar Assad, saying "you shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!'"
Diplomats said the United States was pushing for a vote on Tuesday on its draft resolution setting up a Syria gas attacks inquiry, but Russian Federation could veto the measure.
But Russian Ambassador Vassily Nebenzia has indicated the Kremlin might veto such a move because "it contains some unacceptable elements".
Have there been attacks like this before?
It's believed dozens were killed in Douma.
Syria's conflict erupted with anti-Assad protests in 2011 but has since evolved into a complex war that has killed 350,000 people.
Protests against Bashar al-Assad's regime began in March 2011 as part of the wider Arab Spring which saw governments overthrown in Libya, Egypt and Tunisia.
The Sound and Picture Organization, an activist collective in eastern Syria, said that Iranian fighters and members of Lebanon's Hezbollah group had evacuated their positions in the Boukamal area, near the Iraqi border, which was recently retaken from the Islamic State group.