Prime Minister Theresa May has rebuked US President Donald Trump for suggesting suspects in Friday's London train blast were known to police.
"I never think it's helpful for anybody to speculate on what is an ongoing investigation," she said.
In a tweet, Mr Trump described the attackers as "loser terrorists" and "sick and demented people who were in the sights of Scotland Yard".
The bomb rocked a London Underground train, injuring 29 people.
In a phone call with Mrs May on Friday, Mr Trump "pledged to continue close collaboration with the United Kingdom to stop attacks worldwide targeting innocent civilians and to combat extremism", the White House said.
A Downing Street spokesman said he also offered "his condolences over this morning's cowardly attack in London", which he described to reporters earlier as "a terrible thing".
The call followed Mrs May's critical remarks about the US president.
In one of his tweets, Mr Trump appeared to chide UK authorities: "Must be proactive!"
London police said of his comments, "any speculation is unhelpful".
Another attack in London by a loser terrorist.These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!— Donald J. Trump (@realDonaldTrump) September 15, 2017
Loser terrorists must be dealt with in a much tougher manner.The internet is their main recruitment tool which we must cut off & use better!— Donald J. Trump (@realDonaldTrump) September 15, 2017
Nick Timothy, a former senior aide to British Prime Minister Theresa May, echoed London police's comments.
"True or not - and I'm sure he doesn't know - this is so unhelpful from leader of our ally and intelligence partner," he tweeted.
True or not - and I'm sure he doesn't know - this is so unhelpful from leader of our ally and intelligence partner. https://t.co/ZndrTXFrAX— Nick Timothy (@NickJTimothy) September 15, 2017
Former Conservative MP Ben Howlett also weighed in, calling Mr Trump's tweets "dangerous and inappropriate".
It is highly unhelpful/dangerous and inappropriate for an ally to make announcements that share intelligence and undermine investigations https://t.co/rHcrE8oUNz— Ben Howlett (@benhowlettuk) September 15, 2017
In other tweets on Friday, the Republican president claimed his administration had "made more progress in the last nine months" against the Islamic State group than his predecessor, Barack Obama.
"Must be proactive & nasty!" he tweeted.
Analysis: Changing the subject?
Anthony Zurcher, BBC News Washington
Last month Donald Trump defended his delay in ascribing blame for the violence at a white nationalist rally in Charlottesville by saying: "I wanted to make sure, unlike most politicians, that what I said was correct, not make a quick statement."
That is advice the president seemed uninterested in heeding on Friday morning, as he fired off a series of tweets reacting to the London train incident.
In fact, with few exceptions, Mr Trump is happy to quickly react to international incidents he suspects are perpetrated by Islamic militants, even when - in the case of the Philippines casino robbery - that's not the case.
Besides his London remarks on Friday morning, the president again blasted Senate procedural rules, called for his Muslim travel ban to be "tougher", said the fight against the so-called Islamic State should be "nasty", and slammed ESPN, a US sports cable network, for being too political.
Given that Mr Trump was under fire from his base on Thursday for reaching out to Democrats and being too conciliatory on immigration, it is hard not to think that his actions on Friday may be an effort to change the subject.
Mr Trump also renewed a call to shut down internet capabilities, which he said were a main recruitment tool for terrorists.
During the Republican presidential campaign in 2015, Mr Trump suggested he would ask Microsoft founder Bill Gates to cut off the internet because of terror recruitment.
"We're losing a lot of people because of the internet," Mr Trump said at the time, without elaborating.
"We have to see Bill Gates and a lot of different people that really understand what's happening."
"We have to talk to them about, maybe in certain areas, closing that internet up in some ways," he added.
Friday was not the first time Mr Trump has been accused of reacting to suspected terrorist incidents before the facts are fully known.
If he did leak sensitive information in his tweet about Scotland Yard, rather than indulging in pure speculation as London police say, it could amount to a security breach.
In May, US officials angered British authorities when details about a deadly bomb attack at an Ariana Grande concert in Manchester were divulged to US media.
The New York Times was criticised for publishing photographs from the scene, as well as images of the bomb.
The president also used Friday's tweets to promote his travel ban temporarily barring people from six Muslim-majority countries as well Syrian refugees from entering the US.
The travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!— Donald J. Trump (@realDonaldTrump) September 15, 2017