EU and NATO leaders on Tuesday condemned the twin bombings at the Boston Marathon that killed at least three people and injured more than 140 others.

"I condemn the appalling acts carried out in Boston and deeply regret the tragic outcome and loss of innocent lives. Those responsible for the atrocious acts will be brought to justice," said European Union President Herman Van Rompuy in a statement.

EU foreign policy chief Catherine Ashton said she was "shocked to hear of the terrible bomb attack at the Boston marathon, deliberately targeting men, women and children enjoying and participating in a sporting event."

"Such acts are reprehensible. We stand with the United States government and people in condemning this bombing," she said.

NATO Secretary General Anders Fogh Rasmussen was also "deeply shocked."

"I express my heartfelt condolences to the families and loved ones of those who were killed, and I wish a full recovery to those who were injured. My thoughts are with the people of Boston and all of the United States," he said in a statement.