At the same time, Homeland Security said it did not see an immediate link from the attack toa threat in the U.S.
"The Secretary (John Kelly) has been fully briefed on the incidents and the ongoing response. At this time, we have no information to indicate a specific, credible terror threat in the United States."
The U.S. State Department also condemned the attacks.
"We understand U.K. police are currently treating these as terrorist incidents," State Department spokeswoman Heather Nauert said, in a statement. "The United States stands ready to provide any assistance authorities in the United Kingdom may request."
Metropolitan Police Chief Cressida Dick said the help would be welcomed even after an incident last month at the G7 in which May admonished Trump for an apparent intelligence sharing breach.
"We have a good working relationship with American colleagues," Dick said, adding that normal practice is to work with them and that is the assumption going forward.
In a tweet, U.S. House Speaker Paul Ryan also offered support.
Japan has issued a travel advisory warning its nationals to avoid places of what it calls potential targets in London, including sightseeing spots, department stores, markets, concert venues and political rallies. The Foreign Ministry says Japanese visitors should "pay close attention to the surroundings, and leave quickly if any suspicious signs are seen."
Australian Prime Minister Malcolm Turnbull said that two Australians were directly affected in the attack with one hospitalized as he also condemned the incident.
"This attack is yet another cruel example of the new reality in which we live, the ever-present threat of murderous Islamist terrorists, intent to harm our communities, our way of life and the freedoms we hold dear," Turnbull said.
German Chancellor Angela Merkel said she was dismayed by the attack and that German was firmly at Britain's side.
Russian President Vladimir Putin sent a telegram to May suggesting joint efforts to fight terrorism should be stepped up.
— The Associated Press and Reuters contributed to this report.