New York congressman Anthony Weiner has admitted sending a close-up picture of his crotch to a young woman, but says he will not resign.
The Democratic representative also acknowledged "inappropriate" communications with women online.
Mr Weiner had initially said his Twitter account had been hacked when a photo of a man's crotch in grey underpants was sent from it last week.
On Monday he admitted this was not true, saying that he had "panicked".
A tearful Mr Weiner told a news conference he was "deeply ashamed of my terrible judgement".
"I'm deeply regretting what I have done and I'm not resigning," he said, apologising for any pain caused to his family, constituents, supporters and staff.
He said he had intended to send the close-up picture of himself in his underpants as a direct message on Twitter.
Direct messages can only be viewed by the addressee, whereas normal Twitter messages are generally accessible.
"Once I realised that I had posted to Twitter I panicked, I took it down and said that I had been hacked," Mr Weiner said.
He then said he had had exchanges with women he met online both before and after he married Huma Abedin, a top aide to US Secretary of State Hillary Clinton, last year.
"I have exchanged messages and photos of an explicit nature with about six women over the last three years."
But he said he had not met the women, and had never had sex outside his marriage.
His admission came on the same day a conservative blogger posted what purported to be photos of Mr Weiner without his shirt on.
Andrew Breitbart said he had obtained the photos from a woman with whom Mr Weiner had flirted online.
Mr Weiner, a native New Yorker and former New York City Council member, is known for his outspoken liberal views and his fiery speeches on the floor of the US House of Representatives.
He was widely expected to run for New York mayor in 2013.