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A sixth man was arrested in Britain early on Thursday as part of the probe into the bomb attack on a London Underground train last week, police confirmed.

The 17-year-old was detained under anti-terrorism powers in Thornton Heath, south of London, where a search is under way, a statement said.

Some 30 people were injured in the September 15 attack at Parsons Green station in west London, which was claimed by the Islamic State group.

The bomb had been hidden in a large white bucket and it apparently failed to detonate fully during the morning rush hour but resulted in what witnesses described as a "fireball".

Some commuters suffered burns, while others were injured in the stampede to the exit that ensued. 

Dean Haydon, an anti-terrorism officer, said the investigation was progressing rapidly.

On Saturday, an 18-year-old man, reportedly an Iraqi refugee, was arrested in the ferry departure terminal of the port of Dover and a 21-year-old man was detained later the same day in west London.

A third man, aged 25, was detained by anti-terror officers in Newport, Wales on Tuesday and two more arrests were made in the same city on Wednesday.

The probe has focused on a foster home in Sunbury-on-Thames, a suburb of London where the 18-year-old is believed to have lived.

The bombing was the fifth terror attack in Britain in six months, which combined have claimed 35 lives.

It prompted authorities to raise the terror alert to its maximum "critical" level on Friday, meaning another attack was believed to be imminent, before being lowered back to "severe" two days later.

Despite the Islamic State group's claim of responsibility, both British and US officials have cast doubt on the statement, saying there was no evidence any recognised militant group had ordered or organised the bombing.