He later added: "Loser terrorists must be dealt with in a much tougher manner".
Earlier Friday, Trump posted several tweets about both the terror attack and the ongoing efforts to thwart terrorism.
A police spokesperson told CNN Trump's comment is "pure speculation given we don't know who is involved".
The president weighed in as police in and around London carried out a manhunt for the person or persons who set off a crude incendiary bomb on a subway auto during Friday's morning rush hour.
The tweets followed breaking news that a bomb was detonated on a London tube train on Friday that injured at least 20, including children.
Following the attack, Britain has raised threat level from severe to critical.
But according to Bloomberg, the British prime minister expressed her displeasure with Trump's remarks directly to the president himself. "The travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!" he said.
In a series of Tweets, the president claimed his administration had "made more progress in the last nine months" against the so-called Islamic State than his predecessor, Barack Obama.
President Donald Trump is calling the explosion in London a "terrible thing".
On Friday, he again stressed that the internet was a main recruitment tool for terrorists.
Trump has tweeted his reactions to likely terror attacks in the past. He added, "Must be proactive & nasty!"
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.
Khan, the United Kingdom capital's first Muslim mayor, responded to Trump's tweets on Friday by saying that, "I'm not going to go there".
Trump is using the incident to defend his travel ban against six Muslim-majority countries. In June, he criticized London's mayor, Sadiq Khan, over his response to another terrorist attack, misconstruing Mr. Khan's words.