The teens, aged 15-16, said they watched pornographic films on the internet before riding their motorbikes around the West Lake looking for good looking foreign women. They would approach such women and touch their private parts, the Cong An Nhan Dan (People's Police) newspaper under the Ministry of Public Security reported.

The teens are residents of Cau Giay District and have admitted to their crime, Hanoi police said.

They summoned the teens for questioning after receiving complaints from four foreign women who said they had been insulted and sexually harassed by a group of young men aged 15 to 20 riding motorbikes in the West Lake area.

The case is being investigated further.

Deputy Prime Minister Truong Hoa Binh Friday asked Hanoi authorities and relevant agencies to promptly verify the reports and impose stern penalties on the culprits.

Following the attacks, expats have created a Facebook group called Hanoi Nightwatch as a platform to report cases of sexual assault and suggest ways in which members can protect themselves. It has over 5,000 members.

The U.K. Embassy in Hanoi has also warned British citizens on its official Facebook page: "We are aware that there has been a recent spike in reports of personal attacks, including rape and sexual assault, on foreign nationals in Hanoi. Some reports are of women being victims of indecent assault and harassment including inappropriate touching and groping."

The U.S. Embassy in Hanoi has also urged American citizens to practice vigilance and consider traveling in groups when walking or jogging in the West Lake area, following increased media reports of attacks on foreign women by groups of men on motorbikes.

Some 91,000 foreigners from more than 100 nations and territories live and work in Vietnam, mainly in Ho Chi Minh City and Hanoi.