Russia and Iran have threatened military retaliation against the US, accusing Donald Trump of crossing “red lines” by ordering a cruise missile attack on a Syrian air base.

The two military allies of Syria said the US bombardment had violated international law and, in a statement, added: “From now on we will respond with force.”

The warning came after the Russian embassy in the UK suggested that British and American attempts to deliver an ultimatum to the Kremlin this week could result in a “real war”.