A manhunt was under way in Mumbai on Friday for four alleged members of the same Islamist group that attacked the city in 2008, amid warnings of a strike on foreign targets over Christmas and New Year.

Roads were closed in and around the luxury Taj Mahal Palace hotel — the focus of the deadly siege two years ago that killed 166 - while armed police were out in force at high-profile sites, including overseas consulates.

Mumbai police warned that the four operatives of the Pakistan-based Lashkar-e-Tayyaba (LeT) militant outfit were preparing a "violent attack" during the festive season and called for residents to be vigilant.

"The four recently sneaked into the city to carry out extremely dangerous activity," joint police commissioner Himanshu Roy told a news conference on Thursday evening.

"It is going to be a violent attack." It is the second time this year that city police have issued such a warning.

In September, two Islamist militants were said to be preparing to strike as millions of Hindus thronged the streets of India's financial capital to immerse idols of the popular elephant-headed god, Ganesha, in the Arabian Sea.

No arrests were made. Roy said he had no information about the nationalities of the four men but they were identified as Abdul Karim Musa, Noor Abul Elahi, Walid Jinnah and Mehfooz Alam.

"Special cells" of officers had been formed to "neutralise" the alleged militants and an e-fit of Jinnah had been distributed, he added. "If they have issued an alert of this level it will be credible," homeland security specialist Ajai Sahni, the executive director of the Institute of Conflict Management in New Delhi, told AFP.

"Alerts are issued fairly frequently. They might not have names attached to them as clearly. There seems to be more specific information than there usually is."

The 2008 attacks in Mumbai saw 10 heavily armed gunmen storm three luxury hotels, the city's main railway station, a popular tourist restaurant and a Jewish centre.

India blamed the LeT for training, equipping and financing the operation with support from elements in the Pakistan military and suspended tentative peace talks with its neighbour and long-time rival.

Since then, there have been two high-profile attacks: the first, in Pune, western India, killed 17 in February, while the second, in the holy city of Varanasi earlier this month, left a one-year-old girl dead and over 30 injured.

Both blasts were claimed by the Indian Mujahideen, a home-grown outfit responsible for a series of bomb attacks across India in 2008 and which experts say has links to the LeT and other Islamist groups.

Military analysts have warned that a second serious militant attack emanating from Pakistan against an Indian target could prompt a swift response from India, upsetting regional security.

It could also damage investor confidence in India's fast-growing economy. Mumbai police chief Sanjeev Dayal on Tuesday said that intelligence reports indicated there were "conscious efforts by terror organisations to target foreigners during these two festivals here".

"Extensive security measures" had been put in place and police were "taking things very seriously", he added. One of the news agency quoted official sources as saying the intelligence had specifically mentioned possible infiltration by members of the LeT.

As well as the measures around the seafront Taj hotel, police were conducting surveillance at the nearby Gateway of India monument and plaza, as well as the busy bars and restaurants in surrounding streets. All are popular places for revellers to see in the New Year.