The weapon used in Monday's attack was "amateur-level," New York Governor Andrew Cuomo said at a news conference near the explosion site. He told CNN the homemade bomb may not have fully ignited, possibly limiting the damage.

Officials said the attempted bombing underscored New York City's status as a target for such attacks, citing the attacks of Sept 11, 2001, which killed more than 2,750 people in New York and nearly 3,000 people in all, and the 1993 World Trade Center bombing, which killed six people.

"Let's be clear, as New Yorkers, our lives revolve around the subways," Mayor Bill de Blasio said at a news conference. "And let's also be clear, this was an attempted terrorist attack."

In a statement, President Donald Trump said the attack emphasized the need for US immigration reforms.

"America must fix its lax immigration system, which allows far too many dangerous, inadequately vetted people to access our country," he said.