Israel has conducted an air strike in Syria, apparently targeting a building, according to CNN, although there has not yet been any official confirmation.
The network said US officials did not believe Israel had targeted a chemical weapons facility. CBS News cited US sources as saying Israel targeted a warehouse.
But Syrian UN Ambassador Bashar Ja'afari told Reuters, "I'm not aware of any attack right now."
A White House spokeswoman referred questions on the CNN report to the Israeli government.
The Pentagon declined comment.
In Jerusalem, a Israeli military spokeswoman declined to comment.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.