aircraft

The customs documents obtained by the BBC suggest that Rama Group made two shipments to Moscow of high-end optics that can be used in missiles, tanks and aircraft.

Under the changes, a large attack on Russia with conventional missiles, drones or aircraft could meet the criteria for a nuclear response, as could an attack on Belarus or any critical threat to Russia's sovereignty.

Last week, researchers at the Middlebury Institute of International Studies in California found satellite images that indicate China is working on nuclear propulsion for a new aircraft carrier.

Some travellers commenting on social media said they had been told that pilots were unable to generate loading data for their aircraft and have been struggling to get through to colleagues by phone.

Analysts said the step might give Ukraine more ability to stave off Russian attacks on its cities, but pointed out that Moscow has already moved some aircraft and missile installations out of reach.