The centre of Moscow is enveloped in a mysterious cloud of noxious gas.

The foul smelling vapour spread across the city during the day - forcing residents to stay indoors.

The source of the stench is unclear.

Government officials blamed it on a leak at a local oil refinery, but the owner of the facility has denied any problem at the plant.

The smell is said to resemble rotten eggs - or hydrogen sulphide - which can be highly toxic.