It is being described as the biggest fire in Moscow for decades.A gas pipeline has exploded in the Russian capital, sending flames more than 100 metres into the air. Crowds gathered to watch the blaze, visible for many kilometres around. Some witnesses expressed concerns that the fire could spread further. But an emergencies ministry spokesman said no residential properties had been damaged. Reports say the fire has now been brought under control. However at least five people have been injured. The cause of the blast in the southwest of the city is still unknown. But analysts say ageing pipelines are the most common reason for a rising number of such explosions.