LONDON (Reuters) - Two of Britain’s most senior officers said the pressure on the police forces was not sustainable after last week’s attack on a packed London train became the fifth major attack this year.

Fewer officers could make it harder to prevent future attacks and it will force difficult choices about where to put police resources, they said.

A homemade bomb engulfed a train carriage in flames at Parsons Green underground station in west London last Friday injuring 30. Cressida Dick, London’s police Commissioner, said it could have been much worse.

Britain had previously faced four deadly incidents since March which killed a total of 36 people.

“In the long run, if we continue with this level of threat, which is what people are predicting ... this is not sustainable for my police service,” Dick said in an interview on LBC radio.

Six men have been arrested and four remain in custody since the Parsons Green attack.

“That was a very very dangerous bomb. It partially detonated, it had a large quantity of explosive and it was packed with shrapnel. So it could have been so much worse,” Dick said.

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While the bombing at Parsons Green was not deadly, the aftermath of the attack still saw extra police on the streets and the threat level raised a notch to critical.

Interior minister Amber Rudd has announced an extra 24 million pounds ($32.55 million) of funding for counter-terrorism policing following the bombing, in addition to 707 million previously announced support for 2017/2018.

But while the government has committed to increase the overall spend on counter-terrorism by 3 billion pounds, Sara Thornton, head of National Police Chiefs’ Council (NPCC), said that not enough of the budget would support frontline officers.

There are about 20,000 fewer officers than there were when Prime Minister Theresa May’s Conservatives came to power in 2010 and Thornton said numbers were at levels last seen in 1985 despite a 10 percent rise in crime last year.

“Every time there’s a terror attack, we mobilize specialist officers and staff to respond but the majority of the officers and staff responding come from mainstream policing,” she wrote in a blog post on the NPCC website.

“This puts extra strain on an already stretched service.”