LA PAZ, Jan 25 (Reuters) - Heavy rains and flooding in much of Bolivia have killed 30 people and caused hardship for nearly 25,000 families since November, the Defense Ministry said on Friday, prompting promises of aid from international organizations.

The government declared a national state of emergency this week after flooding worsened.

Several homes were destroyed by mudslides in the La Paz area, and rushing waters killed a father and son in the city's wealthy southern district.

Disaster relief experts from the United Nations were due to start arriving in Bolivia on Friday night to assess the damage and boost aid to South America's poorest country.

The Organization of American States also plans to help in relief efforts, OAS Secretary General Jose Miguel Insulza said on Thursday, during a two-day visit to Bolivia.

The flooding is blamed on the La Nina weather phenomenon in the Pacific Ocean, which has exacerbated Bolivia's rainy season.

Flooding has especially hurt the provinces of Santa Cruz, Beni, Cochabamba and Chuquisaca, wiping out some highways.

Some 24,700 acres (10,000 hectares) of crops have been destroyed and overall damages are estimated at about $30 million, government officials say.

Seventy-one cases of dengue have been confirmed and 700 cases of the tropical disease spread by mosquitoes are suspected, according to the United Nations. (Additional reporting by Stephanie Nebehay in Geneva)

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