Then, a Downing Street spokesman said Trump had offered "his condolences over this morning's cowardly attack in London", which he described to reporters earlier as "a bad thing".
His tweets tend to be aimed at his base, bolstering his image as tough on immigration and terrorism and with little thought to the wider world.
"The police and the security services are doing the work necessary to discover the full circumstances of this cowardly attack that's taken place, and to identify all those who are responsible", she said. "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".
A manhunt is on to find those behind the bombing that wounded 22 people on a Friday morning rush-hour subway train.
Police say 30 people, including a young boy, were injured during the attack on a rush-hour train packed with school children and commuters.
The terror threat level in Britain has been raised to critical, which means a further terrorist attack may be imminent. In response, Prime Minister Theresa May said, "I never think it's helpful for anybody to speculate on what is an ongoing investigation".
"The travel ban into the United States should be far larger, tougher and more specific - but stupidly, that would not be politically correct", the president said in a series of tweets.
Trump later told reporters that the attack "is a bad thing". He added, "Must be proactive & nasty!" The 90-day travel ban on visitors from Iran, Libya, Somalia, Sudan, Syria and Yemen lapses in late September, and the 120-day refugee ban will expire a month later.
Speaking to LBC, London mayor Sadiq Khan said he had been "too busy this morning to look at my twitter" and had not seen Trump's tweets.
Trump is using the incident to defend his travel ban against six Muslim-majority countries.
As for the travel ban, A.P. notes that remains tangled in the court systems with the Supreme Court scheduled to listen to arguments on October 10.