The most recent attack before Friday was a van and knife attack on London Bridge in June.
He then added: "Loser terrorists must be dealt with in a much tougher manner", without giving details of what that meant.
Trump concluded: "Must be proactive & nasty!" "It keeps going and going, and we have to be very smart and we have to be very, very tough perhaps we're not almost tough enough", Trump told reporters in the Rose Garden of the White House. McMaster would not say if Trump revealed privileged information.
Sadiq Khan, the Muslim mayor of London, said that his city "utterly condemns the ugly individuals who attempt to use terror to harm us and destroy our way of life".
Trump turned the London subway bomb explosion into an opportunity to highlight his tough talk on terror and promote his travel ban. McMaster did not characterize the president's conversation with May, saying he was not on the call.
He also suggested that the perpetrator was known to authorities and recruited on the Internet, prompting British Prime Minister Theresa May and a London police spokesperson to publicly rebuke the President.
The tweets followed breaking news that a bomb was detonated on a London tube train on Friday that injured at least 20, including children. "The internet is their main recruitment tool which we must cut off & use better!", POTUS said in an another tweet.
Part of the problem is that Trump lacks the basic impulse control not to tweet. The Supreme Court is scheduled to hear arguments on October 10 on the legality of the bans on travelers from six mostly Muslim countries and refugees anywhere in the world.
The Trump administration's original policy called for a 90-day travel ban - which will be over in late September - and a 120-day refugee ban which will expire a month later.
Trump earlier this year signed an executive order blocking travelers from six majority-Muslim countries, including Syrian refugees. He and his son Donald Trump Jr both mocked Sadiq Khan after taking the mayor's words of reassurance to Londoners out of context.