Several commuters have reportedly suffered burn and stampede injuries after an explosion at Parsons Green underground station in west London.

Witnesses described flames and a “fireball” following the incident at 08:20 BST (12:50 IST)  . Security services and counter-terror teams are monitoring the situation. Armed police, paramedics and firefighters are also on scene.

Flames engulfed one carriage and raced along a train on a west London route to Parsons Green, forcing passengers to trample others as they rushed for an exit, an eyewitness told Reuters.

The London Metropolitan police have declared it a terrorist incident while investigations continue.  They said “a number of people” had been injured and advised people to avoid the area.

Images shared online appeared to show a bucket with wires trailing from it smouldering inside the train.

Prime Minister Theresa May tweeted: “My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident.”  May is to chair a meeting of the Government’s Cobra emergencies committee this afternoon to discuss the terrorist incident at Parsons Green.

Witnesses described the scene of panic as commuters struggled to escape the carriage, “collapsing and pushing” each other.

They described seeing people with burns and, separately, wounds caused when passengers rushed to escape the station.

The United Kingdom terrorism threat level is  now at ‘severe’, meaning an attack is highly likely.

Since March 2017 at least three attacks blamed on an Islamist motivation have claimed lives in Britain, with officials saying that a further six plots have been thwarted. One attack in June was blamed on an extreme right wing motivation.

On Thursday, reacting to news of a 68% increase in terrorism related arrests, the senior national coordinator for counter terrorism policing, Deputy Assistant Commissioner Neil Basu said:“There is no doubt that since March and following the attacks in London and Manchester we have seen a shift-change in momentum. But while the terrorist threat has increased in recent months, so has our activity; reflected by this significant increase in arrests.

“We’re taking every possible opportunity to disrupt terrorist activity – be it making arrests for terrorism offences, intervening where there are signs of radicalisation, or working with communities to prevent terrorists operating in their area.

“Police, together with the security services are determined to make the UK as hostile an environment for terrorists as possible.”

Police say there have been a total of 19 foiled plots since June 2013.