President Barack Obama called Boston mayor Tom Menino and Massachusetts governor Deval Patrick on Monday to offer federal assistance in responding to explosions that rocked the end of the Boston Marathon.
The Secret Service heightened security at the White House, establishing a security cordon on Pennsylvania Avenue outside the front gate of the executive mansion.
A White House official said Obama was notified of the incident around 3 pm EDT (1900 GMT) and was briefed by US, homeland security adviser Lisa Monaco and other members of his senior White House staff in the Oval Office.
"The president called Boston mayor Tom Menino and Massachusetts governor Deval Patrick to express his concern for those who were injured and to make clear that his administration is ready to provide needed support as they respond to the incident," the official said.
The White House referred to the explosions as the "incident in Boston."
Separately, vice president Joe Biden, in a telephone conference call about proposals to tighten gun laws, offered his prayers as he reacted to images he saw on television from Boston.
"As I'm speaking here, they just turned on the television in Washington in my office. Apparently there has been a bombing. I don't know any of the details about what caused it, who did it. I don't think it exists yet," Biden said.
"But our prayers are with those people in Boston who have suffered injury. I don't know how many of them there are. I'm still looking at it on television now," he said.
Biden was speaking off the cuff and did not appear to have any independent knowledge of what caused the explosion.