At least 32 Chinese tourists and four North Koreans have died in a major bus crash in North Korea, China's foreign ministry has said.

Two more Chinese nationals were seriously injured and left in critical condition.

Chinese diplomats have rushed to the scene of the accident in North Hwanghae province, the foreign ministry said.

"We are investigating the cause" of the accident which occurred yesterday, ministry spokesman Lu Kang said, hours after China reported "heavy casualties" in the crash.

Chinese state television's English-language channel reported that a tour bus had fallen off a bridge, killing more than 30 people, but it later deleted the Twitter message.

State television's main Chinese-language news channel showed images of a crashed blue bus with its wheels in the air.

It showed at least one person being treated in hospital, but gave no details of casualties.

In a separate statement, China's health ministry said it was sending a team of medical experts, along with equipment and drugs to North Korea to help treat survivors.

The North Hwanghae province that borders South Korea is home to Kaesong, an ancient Korean capital popular with tourists.

North Korea is a popular destination for Chinese tourists, especially those from the country's northeast.

China said more than 237,000 Chinese visited in 2012, but stopped publishing the figures in 2013.

Chinese visitors make up about 80% of all foreign tourists to North Korea, according to a South Korean think-tank, the Korea Maritime Institute, which estimates that tourism generates revenue of about $44 million each year for the isolated country.

China is North Korea's most important economic and diplomatic backer, despite its anger at Pyongyang's repeated nuclear and missile tests and support for strong United Nations sanctions against North Korea.

North and South Korea are in the final stages of preparations for a summit between North Korean leader Kim Jong-un and South Korean President Moon Jae-in at the border truce village of Panmunjom on Friday.