The source, a counterterrorism official, said U.S. intelligence had indicated a group might enter the country from the seas to attack Mumbai, the country's major port and the financial capital.

The gunmen, who went on a massacre spree last Wednesday before their siege ended three days later, are widely believed to have entered Mumbai using rubber boats after traveling in a trawler. About 180 people died and more than 200 injured in the attacks.

The CNN report said Indian security forces confirmed the U.S. warning, noting it was actually issued twice. The officials said a higher alert was later placed at the likely areas of entry but was eventually reduced.

Separately, some Mumbai fishermen have claimed sending information about seeing a group of gunmen arriving by boat.

CNN's sister station CNN-IBN quoted sources as saying officials had recovered phones and a global navigational device on an abandoned boat off the coast of Mumbai. The body of the captain of the boat, which had been hijacked, was found with his hands tied behind his back.

India has claimed the attacks were planned in Pakistan by the Lashkar-e-Taibas, a group linked to al-Qaida. Pakistan said it had no evidence about any such involvement.