Sarah Palin, vice-presidential running mate for Republican presidential candidate John McCain, and her husband have announced their 17-year-old daughter is pregnant. (Marc Lester/Anchorage Daily New/Associated Press)

The woman running on the Republican ticket for U.S. vice-president announced Monday that her 17-year-old, unmarried daughter is pregnant.

Sarah Palin and her husband Todd released a statement on presidential candidate John McCain's website saying daughter Bristol will marry the father.

"Our beautiful daughter Bristol came to us with news that as parents we knew would make her grow up faster than we had ever planned," they said.

"We're proud of Bristol's decision to have her baby and even prouder to become grandparents."

The family will support the daughter, they said, adding that the couple is "going to realize very quickly the difficulties of raising a child."

They also asked the media to respect the family's privacy.

The website Daily Kos alleged in a post on Sunday that Sarah Palin had faked her most recent pregnancy to cover up the fact that her youngest child, four-month-old Trig, was actually her daughter's illegitimate baby.

The site used photographs and video to suggest that Palin's surprise announcement of her pregnancy last February, while she was apparently in her seventh month, was an attempt to avoid the embarrassment of a media frenzy about teenage pregnancy in the Alaska governor's mansion.

'Life happens,' McCain aide tells press

A spokesman for the Palin family, Bill McAllister, denied the allegations. As well, an adviser to McCain, Mark Salter, said the announcement of the daughter's pregnancy was being made to rebut such rumours.

Salter and fellow McCain aide Steve Schmidt said the Arizona senator had known about Bristol's pregnancy before her mother was offered the Republican vice-presidential nomination.

"Senator McCain's view is this is a private family matter. As parents, [the Palins] love their daughter unconditionally and are going to support their daughter," said Schmidt.

"Life happens," he added.

Democratic candidate Barack Obama has denied that his campaign had anything to do with the web rumours about Bristol Palin and has said he will fire any member of his team who spreads such information.

"People's families are off limits, and people's children are especially off limits," Obama told reporters during a whistle stop in Michigan.

The announcement of Bristol Palin's pregnancy comes as the Republicans gather to nominate McCain and his vice-presidential running mate in St. Paul, Minn.

Senior U.S. political analysts are already discussing whether the news will help or harm Republican attempts to emphasize Sarah Palin as a serious contender for vice-president.

She has been criticized by members of Obama's campaign for lacking experience, but early polls indicate Palin's open adherence to evangelical Christianity is playing out well among social conservatives who have — in the past — been skeptical about McCain.