Shanghai World Expo organizers haven't received a letter of confirmation from the United States on its participation in and the appointment of a commissioner general to the 2010 event.

Countries and organizations are only regarded as attending the World Expo when the host receives official confirmation, according to the International Exhibition Bureau.

So far, 191 countries and 48 international organizations have confirmed their participation, said the Bureau of Shanghai World Expo Coordination.

The U.S. government has only promised orally to take part in the six-month-long event that starts on May 1 and ends Oct. 31 next year.

The bureau said it has been keeping close contact with the U.S. preparatory team and the U.S. side has made breakthroughs in its fund-raising efforts.

U.S. Secretary of State Hillary Clinton announced July 1 Washington local time that Jose H. Villarreal was named as U.S. Commissioner General to the 2010 World Expo.