US Secretary of State John Kerry condemned the attack against Iranian assets, calling the bombing "senseless and despicable terrorism."
"The United States knows too well the cost of terrorism directed at our own diplomats around the world, and our hearts go out to the Iranian people after this violent and unjustifiable attack claimed the life of at least one of their diplomats," Kerry said in a statement.
The timing of the overture to Iran – just a day before talks were to resume in Geneva over Iran's controversial nuclear program – was questioned by journalists at the daily State Department press briefing with spokeswoman Jen Psaki.
"I think this was just obviously a tragic event that was important to condemn, and send that strong message," Psaki said, adding that the US position on Iran being the world's leading state sponsor of terror "has not changed."
United Nations Secretary-General Ban Ki-moon condemned the attack, extending his condolences to the governments of Iran and Lebanon and calling for "expeditious" justice.
"The Secretary-General calls on all in Lebanon to recognize that such appalling and indiscriminate acts of violence target all in Lebanon," Ban's spokesman said in a statement. "He urges all Lebanese parties to act with restraint and to support the institutions of the state, and particularly the security forces, as they seek to prevent further acts of terrorism."