Major airports across the country were put on high alert after receiving the information from intelligence agencies about possible aerial strikes.
Air force chief Fali Homi Major told the Press Trust of India news agency: "This is based on a warning which has been received and we are prepared as usual."
Indian Defense Minister A.K. Antony summoned his military chiefs Wednesday to warn them to be prepared for attacks from the air and the sea in the wake of growing criticism about slack security after the Mumbai attack last week.
Meanwhile, Mumbai police said an unexploded hand grenade was found Thursday outside a hospital that was the scene of an attack during last week's siege on the city.
Senior Police Inspector Shashi Pal said the grenade was found by a patient in a box of garbage behind Cama Hospital. The bomb squad was quickly called to the scene.
Pal said the grenade may have been left by the gunmen, but an investigation has not yet been completed.
Police, almost a week after they were left there by gunmen behind the attacks - a stunning new example of the botched security that has sparked outrage in India since the deadly three-day siege.
While searching through a mound of about 150 bags, which police believed were left by the dozens of victims in the Chhatrapati Shivaji Terminus station, an officer found a suspicious-looking bag and called the bomb squad, said Assistant Commissioner of Police Bapu Domre. Inside were two 8.8-pound bombs, which were taken away and safely detonated, he said.
After the attacks, police found unexploded bombs at several of the sites, including two luxury hotels and a Jewish center.
It was not immediately clear why the bags at the station were not examined earlier. The station, which serves hundreds of thousands of commuters, was declared safe and reopened hours after the attack.
The discovery has added to increasing accusations that India's security forces missed warnings and bungled its response to the Nov. 26-29 attacks.
Indian navy chief Sureesh Mehta has called the response to the attacks "a systemic failure." The country's top law enforcement official has resigned amid criticism that the 10 gunmen appeared better coordinated and better armed than police in Mumbai.
India continued directing the blame Thursday for last week's three-day terrorist attack in Mumbai at a banned Pakistani militant group, with an intelligence official claiming two of the groups leaders had masterminded the siege.
Secretary of State Condoleezza Rice told Pakistani leaders Thursday during a visit to the country that the U.S. wanted to see ain response to the Mumbai attack, going beyond just another ban on Islamic groups, reported CBS News Farhan Bokhari.
Her meeting with Pakistan's president did yield an official response from the nation which now feels the accusatory stare of most of the Western world - and India - over the Mumbai attack.
CBS News has learned that the FBI and Department of Homeland Security issued a new bulletin on Thursday to American building owners and operators, as well as law enforcement, to alert them to the latest information on the Mumbai attacks, some of it from uncorroborated open sources and some from interrogations with the captured terrorist.
The agencies "have no credible or specific information that terrorists are planning operations against public buildings in the United States."
Pakistani President Asif Ali Zardari said he would take "strong action" against elements in his country that were involved in the 62-hour terrorist siege.
Pakistan has complained that India has shared no evidence linking it to the attacks.