US Senator Al Franken has responded to a woman's allegations that he groped her as she slept and "forcibly" kissed her in a rehearsal for a comedy skit.

Leeann Tweeden says the two incidents happened in December 2006 on a tour to entertain US troops overseas, before Mr. Franken entered politics.

The radio host wrote that the former comic "aggressively" kissed her while saying they had to rehearse a scene.

Mr. Franken, a former Saturday Night Live writer, apologized for the grope.

Image copyright KABC

Image caption Franken said the photo "was clearly intended to be funny but wasn't"

But the Minnesota Democrat said he has a different recollection of the other incident.

"I certainly don't remember the rehearsal for the skit in the same way, but I send my sincerest apologies to Leeann," he said in a statement.

"As to the photo, it was clearly intended to be funny but wasn't. I shouldn't have done it."

Mr. Franken issued a second, longer statement less than two hours later after a ferocious backlash from critics who accused him of a non-apology and demanded his resignation.

Image caption Pentagon photos of the 2006 Hope & Freedom Tour in Kuwait show the two performing a skit

In an article for KABC, a Los Angeles radio station where Ms. Tweeden now works, she recalled feeling victimised by Mr. Franken during her ninth USO tour of the Middle East.

"When I saw the script, Franken had written a moment when his character comes climbing at me for a 'kiss'," she said.

"I suspected what he was after, but I figured I could turn my head at the last minute, or put my hand over his mouth, to get more laughs from the crowd."

On the day of the performance in Kuwait, she said he was insistent they rehearse the skit.

Image caption Ms. Tweeden signs autographs at her 2007 tour

But when the moment came he "put his hand on the back of my head, mashed his lips against mine and aggressively stuck his tongue in my mouth".

"You knew exactly what you were doing," she wrote. "You forcibly kissed me without my consent, grabbed my breasts while I was sleeping and had someone take a photo of you doing it, knowing I would see it later, and be ashamed."

Later at a press conference in Los Angeles, Ms. Tweeden went into further detail about how "he stuck his tongue in my mouth so fast".

"And all I can remember is that his lips were really wet and slimy," she said.

Image copyright Getty Images

Image caption Al Franken has been married to his wife, Franni (R), for more than 40 years and they have two adult children

"In my mind, I called him 'fish lips' the rest of the trip."

The Democratic leader of the Senate promptly called for the chamber's Ethics Committee to investigate Franken.

Senator Chuck Schumer said in a statement: "Sexual harassment is never acceptable and must not be tolerated."

The Republican Senate leader Mitch McConnell echoed the call for an ethics investigation.

Mr. Franken said he would "gladly cooperate".

In his second statement, he said: "I respect women. I don't respect men who don't.

"And the fact that my own actions have given people a good reason to doubt that makes me feel ashamed."

Of the photo, he added: "I look at it now and I feel disgusted with myself. It isn't funny. It's completely inappropriate.

"It's obvious how Leeann would feel violated by that picture."

But he still begged to differ on her portrayal of him forcing his tongue down her mouth.

"While I don't remember the rehearsal for the skit as Leeann does," he said, "I understand why we need to listen to and believe women's experiences."

As a senator for Minnesota, Mr. Franken has cultivated an image for himself as a vocal champion of women's issues, from sexual assault to abortion rights and equal pay.

Ms. Tweeden said she was inspired to come forward more than a decade later after California Congresswoman Jackie Speier was interviewed on her morning radio programme.

Mrs. Speier told about being sexually assaulted as a young congressional aide in Washington.

She also claimed to know two currently serving members of Congress who "have engaged in sexual harassment", though she did not name them.