The fact that in the three years until March this year, British police foiled 13 potential attacks is an encouraging signal. "Must be proactive!" he wrote in his first tweet.
"Another attack in London by a loser terrorist", Trump added.
Trump earlier this year signed an executive order blocking travelers from six majority-Muslim countries, including Syrian refugees. Along with advocating for a "larger, tougher" travel ban, the President also suggested shutting down the internet.
Trump followed minutes later with more tweets that talked about the fight against terrorism and his administration's efforts to do so.
"I'm going to call the prime minister right now", Trump said.
The President added: "America and our allies will never be intimidated". But none of the injuries were life threatening.More news: Idaho AG warns consumers in wake of Equifax hack
A bomb partially exploded on a London train during the morning rush hour on Friday, injuring 30 people and prompting the government to raise the country's terrorism threat level to "critical".
Trump called the British prime minister to offer his condolences over the attack on a subway train, which injured 22 people.
The president has repeatedly criticized London Mayor Sadiq Khan, the British capital's first Muslim leader.
The whole event was a little surreal: the President had gone outside to watch an 11-year-old called Frankie Giaccio mow the lawn (Frankie had written to the White House asking if he could) and ended up talking worldwide affairs.
He termed Trump's comments unhelpful from Britain's intelligence partner.
"For years, Scotland Yard has been a leader, as our Federal Bureau of Investigation has been a leader", McMaster said, conciliatorily.More news: Everton suffers dismal defeat against Atalanta
His tweets about the London attack will attract similar accusations of carelessness.
"I think we're always behind the curve".
Pressed to explain further, McMaster said that United States and UK agencies are both working to prevent terrorism.
Watch and see if you can guess what comes next. "It is likely that law enforcement had been working on that problem set".
Following Friday's explosion, Trump took to Twitter to condemn the "loser terrorist" behind the attack, making assumptions that the individual was likely associated with ISIS. After two terror attacks in the city in June, Trump seized on the moment.More news: Jennifer Lawrence reveals perils of breastfeeding a robot baby in 'mother!'