The advisory steps up a warning issued last month
The US Embassy in India has warned of possible "imminent attacks" by militants in the capital, Delhi, and advised US citizens to be vigilant.
"There are increased indications that terrorists are planning imminent attacks in New Delhi," said a statement on the embassy's website.
It urged people to avoid several locations popular with Westerners.
Militants linked to Pakistan-based Islamist groups have been blamed for previous attacks in the city.
The advisory, which steps up a warning issued last month, said that popular shopping areas such as Chandni Chowk, Connaught Place, Greater Kailash, Karol Bagh, Mehrauli, and Sarojini Nagar, should be avoided.
It said US citizens travelling or living in India should maintain "a high level of vigilance", and be aware of their surroundings or any unattended packages.