President Donald Trump used the attacks in London to renew his controversial campaign for a travel ban.
An hour after reports of a van ploughing into people on London Bridge, the US president tweeted: "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!"
Mr Trump initially retweeted a Drudge Report item about the attacks, then provided his own message about the travel ban.
A few minutes later he tweeted a message of support for Londoners: "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!"
Earlier this week, the Trump administration asked the Supreme Court to immediately reinstate its ban on travellers from six mostly Muslim countries and refugees from anywhere in the world.
The administration argues that the US will be safer if the policy is put in place.
Lower courts have blocked the policy, citing various reasons including statements Mr Trump made during the 2016 campaign.
The legal fight pits the president's authority over immigration against what lower courts have said is a policy that purported to be about national security but was intended to target Muslims.
Mr Trump later called Prime Minister Theresa May to offer his condolences.
The White House said the president "praised the heroic response of police and other first responders and offered the full support of the United States government in investigating and bringing those responsible for these heinous acts to justice".
The US State Department issued a statement condemning the attacks as "cowardly".
"The United States stands ready to provide any assistance authorities in the United Kingdom may request," a State Department spokeswoman said.
"Our hearts are with the families and loved ones of the victims. We wish a full and quick recovery to those injured in the attacks. All Americans stand in solidarity with the people of the United Kingdom," she said.