Indian Prime Minister Manmohan Singh Monday expressed deep shock and sadness over the allegedly racist attack upon a Sikh temple or gurdwara in the U.S. state of Wisconsin on Sunday, in which six people of Indian origin were killed.

Singh also hoped the U.S. authorities would probe the attack and ensure that such "ghastly events" did not recur.

"It is a very sad occurrence, the senseless killing of people who gathered at a religions congregation. It happened under tragic circumstances, it makes me enormously sad," said Singh in a statement.

The U.S ambassador to India Nancy Powell called on Sikh leaders at a gurdwara or Sikh temple in the heart of the capital and offered her condolences, said Indo-Asian News Service.

Indian Minister of State for External Affairs Preneet Kaur also said the security of gurdwaras in the United States should be taken care of.

An ex-U.S. army officer opened fire indiscriminately upon people in a Sikh temple in Wisconsin. Police said the attacker, who was killed by police, could have acted out of racist motives.