NEW DELHI: A US woman has told police she was gang-raped at a top hotel in the Indian capital during a visit earlier this year -- the latest in a series of alleged sexual assaults against tourists.

Delhi police launched a formal investigation Saturday after receiving a direct complaint from the woman, who is in the United States and had initially contacted them through an email by a US-based NGO.

"The police have registered a case of rape and will now verify all aspects of the woman's complaint," Joint Commissioner of police Mukesh Meena told AFP.

"We had received the email (from the US-based NGO) earlier but could register a case only after getting a direct complaint from the woman late Saturday," the officer added.

Foreign Minister Sushma Swaraj urged police to "bring guilty to book" after initial media reports of the incident.

"I have also asked Indian Ambassador in US to contact the victim and assure her that we will not spare the guilty," she tweeted on Saturday.

Local media reports, citing police sources, said the woman had travelled to India in April but cut her visit short after the alleged crime.

The woman has complained that her local tour guide, who had access to her room, gave her a spiked bottle of water and raped her with four other acquaintances inside her hotel room.

"She said that after having water from the bottle, she started feeling dizzy. Then four other men... entered the room and locked it," a report in the Sunday Times of India said.

Police are now verifying the woman's timeline and tracking the suspects, including by checking security camera footage from the hotel.

Sexual attacks on tourists in India are widespread, with several western countries warning visitors about the risk.

A 35-year-old Japanese tourist was raped in southern India last month.

Last year a Japanese woman was drugged and raped by a tourist guide in the western city of Jaipur, less than a month after six men gang-raped a 22-year-old Japanese tourist in the eastern city of Kolkata.

An Indian court in June this year convicted five men for gang-raping a 52-year-old Danish tourist in New Delhi in 2014. They were jailed for life.