"Our hearts bleed for the precious and innocent lives lost in Oak Creek," a suburb of Milwaukee, Indian Ambassador Nirupama Rao wrote on Twitter.
"This is a very tragic time for our community. We must remain calm," she said.
"The Sikhs are among the best Indian Americans -- patriotic, law-abiding, industrious, generous and giving. I grieve with them today," she said.
Rao said she received telephone calls from John Brennan, the anti-terrorism adviser to President Barack Obama, and Wendy Sherman, the number three at the State Department, to condemn the attacks.
In a modern twist to diplomacy, the Washington-based ambassador of India's historic rival Pakistan, Sherry Rahman, took to Twitter to voice sadness over the massacre.
The Indian embassy in Washington said in a statement that it was monitoring the incident, which left at least six people dead.
The embassy said that it was in touch with the White House's National Security Council and that a diplomat had been sent to the scene.
Sikhism originates in India. While the gunman was not immediately identified, the US Sikh community -- estimated at 500,000-700,000 strong -- has faced attacks in the past by assailants who mistakenly believe they are Muslim.
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