"Another attack in London by a loser terrorist".
"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.
The most recent attack before Friday was a van and knife attack on London Bridge in June.
In a series of early-morning Twitter posts, Mr. Trump said the London subway attack had been the work of "a loser terrorist" and suggested that those behind the assault had been "in the sights" of the British authorities.
An improvised explosive device (IED) went off on an underground train during the morning rush hour at a busy station in London, leaving at least 22 people injured, in what Scotland Yard called a "terrorist incident".
"The attack was meant to cause significant harm, and 22 people have been injured and are being treated in hospital", May said while speaking to media.
"What the President was communicating is that obviously all of our law enforcement efforts are focused on this terrorism threat for years", McMaster said.
Meanwhile, the U.S. president on Friday claimed his administration had "made more progress in the last nine months" against the Daesh (ISIL) terrorist group than his predecessor, Barack Obama. He and his son Donald Trump Jr both mocked Sadiq Khan after taking the mayor's words of reassurance to Londoners out of context. He added, "Must be proactive & nasty!"
Trump, addressing reporters briefly on camera, called the attack "an absolutely bad thing". 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.
And the White House said Mr Trump had pledged close collaboration with the United Kingdom "to stop attacks worldwide targeting innocent civilians and to combat extremism". "The internet is their main recruitment tool which we must cut off & use better!" he added, without elaborating.
The U.S. Supreme Court is expected to hear oral arguments to decide the travel ban's constitutionality in October.