NEW YORK - US Secretary of State Hillary Clinton on Monday said her "lips are sealed" over daughter Chelsea's ultra-secret upcoming wedding.

Almost nothing is known about Chelsea's marriage to investment banker Marc Mezvinsky except that it takes place July 31 and seems sure to be the society event of the summer.

Asked about the guest list, Clinton told NBC television: "My lips are sealed. I am under very strict orders not to talk about it, and I think for the right reasons, because it is hers, and it is a family wedding."

Even the location remains unconfirmed, not to mention 30-year-old Chelsea's dress designer.

Gossip blogs and people watchers are putting their money on luxury Astor Courts, modeled after France's Versailles palace, in Rhinebeck, upstate New York.

But some 400 guests -- expected to be a glitzy combination of Washington and Hollywood, including TV chat show queen Oprah Winfrey -- will reportedly receive instructions no more than a week ahead.

President Barack Obama is not expected to attend, although initially the grapevine said he would. "Not that I'm aware of," Obama's spokesman said this month when asked to clarify.

The secretary of state, on a trip to troubled US ally Pakistan, told NBC only that "the people coming are her friends and people who have been meaningful in her life."

The Clinton family is also taking pains to emphasize that Chelsea's big day won't be overshadowed by her powerful mother or probably even more famous father, ex-president Bill Clinton.

Hillary Clinton said her husband was just hoping he could perform his paternal duty for their only child.

"You should assume that if he makes it down the aisle in one piece it's a major accomplishment. He's going to be so emotional, as am I," Hillary Clinton laughed.

She did not address another intriguing question: whether Chelsea Clinton, a Christian, and Mezvinsky, a Jew, will be married by a priest, a rabbi, both -- or in a purely secular ceremony.

Instead, Clinton used the mixed-marriage theme as a plug for what she called "this wonderful experiment known as America."

"Over the years, so many of the barriers that prevented people from getting married, crossing lines of faith or color or ethnicity have just disappeared," Hillary Clinton said. "I think in the world that we're in today we need more of that."

If Bill Clinton is worried about nerves, Hillary has publicly agonized over what to wear.

"If you don't tell anybody..., we're still looking," the secretary of state said on CNN in February. "I don't have a dress yet, no, and Chelsea doesn't either."

The consensus of Chelsea Clinton watchers is that the former First Daughter will sport something by leading fashion designer Oscar de la Renta. Well, either that or Vera Wang.

The wedding has been shrouded in mystery ever since the engagement, which was announced last November in an e-mail.

At the time, the couple had been widely reported to be readying to marry in Martha's Vineyard, a longtime playground for the politically connected on the picturesque New England coast.