Trump offers help on London attacks, touts need for travel ban

British police rushed to two incidents in central London after a van ploughed into pedestrians on London Bridge and reports emerged of multiple stabbings in the nearby Borough Market area.

Police said the attacks had been declared terrorist incidents.

"Whatever the United States can do to help out in London and the U.K., we will be there - WE ARE WITH YOU. GOD BLESS!" Trump said in one of two tweets.

"We need to be smart, vigilant and tough. We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!" he said in the other.

Trump's appeal for his travel ban followed his emergency request that the Supreme Court reinstate the executive order that would bar people entering the United States from six predominantly Muslim countries. It has been blocked by lower courts.

The U.S. State Department condemned what it called "the cowardly attacks targeting innocent civilians" in a statement, and echoed Trump's readiness to provide any assistance that British authorities request.

"All Americans stand in solidarity with the people of the United Kingdom," State Department spokeswoman Heather Nauert said in the statement.

Trump was briefed earlier about the London Bridge incident by his national security team, according to White House spokesman Sean Spicer, who said on Twitter that security officials would continue providing the president with updates.

The State Department also said it was monitoring the situation in London closely, and advised American citizens in Britain to heed the advice of local authorities and maintain their security awareness.

Law enforcement officials in major U.S. cities said they were not aware of any threats but were on alert following the attacks in London. "There is no credible threat or nexus to LA," the Los Angeles Police Department tweeted. "We ask Angelenos to stay vigilant."

The New York Police Department's Counterterrorism Bureau said on Twitter that its critical response teams had been deployed to heavily travelled pedestrian areas.

"Go about your Sat. night, NYPD cops are protecting you. Our prayers are with all in London," the police department's official news account tweeted, along with a picture of three heavily armed officers and a police dog in Times Square.

Trump says the travel ban, a centrepiece of his 2016 campaign, is needed to protect Americans from terrorist attacks. Critics say his reasoning is flawed and assail the ban as discriminatory.