One day after Britain said it had foiled a major plot to bomb trans-Atlantic airliners, the US embassy in New Delhi said that Islamists may also be planning a series of blasts in the run-up to India's Independence Day on August 15.
"The embassy has learned that foreign terrorists, possibly including members of al-Qaeda, allegedly intend to carry out a series of bombing attacks in or around New Delhi and Mumbai in the days leading up to India's Independence Day ...," the embassy said in a statement on its website.
"Likely targets include major airports, key central Indian government offices, and major gathering places such as hotels and markets," it said.
Police in New Delhi also stepped up security, including at the Indira Gandhi International Airport.
"The Delhi police backed by central intelligence and paramilitary agencies are maintaining the highest state of alert as part of our Independence Day security drill," a senior official from the federal home ministry told AFP.
"All vital installations like bus and railway stations, airports, metro services and government buildings are under the scanner," he said.
Police said security was also raised in the eastern city of Kolkata, where five guards of the American Centre were killed when Muslim militants attacked the facility in January 2001.
Serial train bombings in Mumbai
killed 182 people on July 11
US embassy spokesman David Kennedy told CNN-IBN news channel that the information was based on intelligence inputs gathered along with the Indian government.
But Indian officials said that the US had not formally spoken to the government about the threat and said the embassy's advisory was "innocuous".
"I am not aware of any formal communication. Our intelligence agencies are already on alert," Anand Sharma, the Indian junior foreign minister, said in New Delhi, referring to the increased state of alert ahead of Independence Day.
"We are aware that there is a threat, as India has been a target for militants," he said.
The home ministry's top official said the alert should be treated only as an advisory. "It is a very innocuous advisory. And that's about it," VK Duggal, the home secretary, said.
"I have read the advisory. It's an advisory to their own staff, we have no comments."
Indian officials said, however, that security had been further tightened in the capital New Delhi and the financial hub Mumbai, capital of western Maharashtra state, with extra protection ordered for key facilities.
"Not only airports but we have information about other vital public installations which could be targeted," AP Sinha, Maharashtra home secretary, told AFP by telephone from Mumbai.
"We had been maintaining a high state of alert but after what happened in London yesterday we further intensified security."
London police said on Thursday
it thwarted a major terror plot
Britain and the US on Thursday sounded their highest state of alert after London police thwarted plans to bomb US-bound airliners.
Police, meanwhile, announced the overnight arrest of two men at New Delhi railway station allegedly carrying a quantity of explosives.
Police said the pair, one of them a Pakistani national, had links with the Lashkar-e-Taiba Muslim group which is engaged in guerrilla warfare with Indian soldiers in the divided Kashmir province.
The US warning comes exactly a month after serial train blasts in Mumbai killed 183 commuters and injured nearly 900.