Washington, Sep 16 (IANS): US President Donald Trump pledged close collaboration with Britain, hours after his tweet on the latest London attack drew pushback from the Britain's Prime Minister Theresa May.

In a phone call with May on Friday, Trump conveyed his sympathies for those injured in the London terrorist attack, saying that the two countries will continue close collaboration to combat extremism, Xinhua news agency cited a White House statement.

On Friday morning, Trump indicated that he expected harsher penalties for terrorists. "We have to be very smart and we have to be very, very tough -- perhaps we're not nearly tough enough," he told reporters at the White House.

Trump's words came hours after his Friday morning tweet, which called the explosion another attack "by a loser terrorist".

He also tweeted that "these are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"

Asked about Trump's comments later by British media, May said in London that "I never think it's helpful for anybody to speculate on what is an ongoing investigation."

A blast took place on a packed rush-hour carriage at Parsons Green subway station on Friday morning in London, leaving at least 29 people injured.

The police has identified the explosion as a terrorist attack.

The terror threat level in Britain has been raised to critical, which means a further terrorist attack may be imminent.