As the death toll of Saturday's explosion in New Delhi's Mehrauli market rose to three Sunday, the U.S. ambassador to India expressed condolences to the Indian people.
David Mulford said he was shocked and saddened by the "senseless" bombing.
U.N. Secretary-General Ban Ki-moon says he is deeply concerned about the string of bombings targeting civilian areas in India.
The latest blast hit a crowded market in India's capital Saturday, instantly killing a 13-year-old boy and wounding at least 20 other people.
Police say witnesses saw two men on a motorcycle drop a black plastic bag outside a shop at the Mehrauli market.
The witnesses said the boy picked up the bag and was trying to return it to the men when it blew up. The suspects fled.
No one has claimed responsibility for the attack.
Saturday's explosion came two weeks after a series of bombings killed 22 people in markets across New Delhi.
India's Foreign Minister Pranab Mukherjee says the government will take strong measures to put an end to such acts of terror.
An Islamist militant group, Indian Mujahideen, said it was responsible for the September 13 bombings in New Delhi.
The group also says it was behind a series of July bombings that killed at least 45 people in Gujarat.
Police say Indian Mujahideen has ties to a Pakistan-based militant group, Lashkar-e-Taiba.
Pakistani Prime Minister Yousaf Raza Gilani condemned Saturday's blast and expressed grief at the loss of life. He also said Pakistan firmly believes such acts of terrorism are against humanity.
Some information for this report was provided by AP.