Donald Trump has warned military action is imminent in Syria, declaring missiles "will be coming".
The US president has been in talks with Britain and France to secure a joint response to the suspected chemical weapons attack in the rebel-held town of Douma.
Russia has warned against military action and said it will shoot down rockets fired at its ally.
In a tweet responding to the threat, President Trump said: "Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and "smart". You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!"
He added: "Our relationship with Russia is worse now than it has ever been, and that includes the Cold War. There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together. Stop the arms race?"
Russia's ambassador to Lebanon, Alexander Zasypkin, had warned that the federation's military will shoot down US rockets and "even the sources that launched the missiles".