CIA Director and secretary of State-designate Mike Pompeo was also spotted at the White House on Wednesday.
A spokeswoman for Air France said the airline had changed some flights paths following the warning, including for Beirut and Tel Aviv flights, while budget airline easyJet said it would also re-route flights from Tel Aviv. He said any action would "target chemical weapons" stocks.
Cancels planned Lat-Am summit trip later this week to 'oversee' US response.
"As the President noted on April 8, the chemical weapons attack by the Syrian regime against innocent civilians.was horrifying, and demands an immediate response from the international community", Pentagon spokesman Eric Pahon said in the statement.
"He's doing exactly what he criticized Obama for doing", Mara said on Morning Edition.
Eurocontrol's warning cited a document from the European Aviation Safety Agency (EASA), Europe's safety regulator. They both blocked each other's proposals to set up independent investigations.
Russia had proposed setting up an investigative panel under rules that would have required Security Council approval of its findings - a measure the West charged would compromise its independence.
Twelve countries voted in favour, Bolivia joined Russia in voting against the resolution and China abstained.
Last night, Mrs May and Mr Trump vowed to stop Assad using chemical weapons again.
"We should all be outraged at these horrific reports and images from Douma", said Dr Peter Salama of the WHO.
US envoy Nikki Haley responded by calling the vote a "travesty". It was not immediately clear whether that visit might affect any U.S. or allied military action.
Will there be a military strike?
Qatar hosts the United States' main air operations center for the Middle East, which would coordinate any American air attack in Syria.
It could be argued a strike is being carried out in support U.N. Security Council resolutions, as was the case in 1998 when a U.S. and British bombing campaign hit Iraqi weapons research and storage facilities to retaliate for the Iraqi government's refusal to cooperate fully with U.N. weapons inspectors.
The Syrian government and Russia, however, say the reports are bogus.
It said this included breathing difficulties, bluish skin, mouth foaming, corneal burns and "the emission of chlorine-like odour".
The U.S. Energy Information Administration (EIA) said on Tuesday that it expects domestic crude oil production in 2019 to rise by more than previously expected, driven largely by growing U.S. shale output.