Armed police rushed to an incident at a London underground station on Friday with local media reporting there had been an explosion on a packed rush-hour commuter train.

Emergency services said they had been called to reports of an incident on a train at Parsons Green station in West London at 8.20 a.m. (0720 GMT) but had no further details of what had happened.

The Metro newspaper reported that passengers had suffered facial burns from a blast and others had been hurt in a subsequent stampede.

"I was on second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me," a man who was on the train told Reuters.

Outside the station, a woman was sitting on a pavement with a bandage around her leg, while armed police patrolled. A Reuters witness saw a woman being carried off on a stretcher with her legs covered in a foil blanket.

"We are aware of an incident at #ParsonsGreen tube station. Officers are in attendance," London police said on Twitter. "We would advise people to avoid the area."

An unverified picture circulating on social media showed a white bucket in a supermarket freezer bag with what appeared to be wires coming out of the top on the floor of one train carriage.

Police said they were aware of reports on social media and said they would release facts regarding the incident once they could be sure of their accuracy.

A Reuters witness could see a bomb disposal unit at the scene while the fire brigade said it had sent six engines and 50 firefighters. London Ambulance said it had sent "multiple resources" including its hazardous area response team to the scene.

"Our initial priority is to assess the level and nature of injuries," it said.

Transport for London said on Twitter there was no service on the western part of the District Line which runs through Parsons Green.

Britain has suffered four attacks blamed on terrorists so far this year which killed 36 people. In 2005, 52 people were killed when four British Islamists carried out suicide bomb attacks on three London underground trains and a bus.