Washington: US President Donald Trump has taken to Twitter amid the unfolding deadly drama in London to offer US help to Britain and to promote his controversial travel ban as an extra level of security for Americans.

British police rushed to 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 UK, we will be there - WE ARE WITH YOU. GOD BLESS!" Trump said in the second of two tweets.