SEOUL (Reuters) -- North Korean leader Kim Jong Un visited the Chinese embassy in Pyongyang on Monday to express "deep sympathy" over a bus accident that killed 32 Chinese tourists and critically injured two, the North's state media said on Tuesday.

"He said that the unexpected accident brought bitter sorrow to his heart and that he couldn't control his grief at the thought of the bereaved families who lost their blood relatives," the North's central news agency said.

The accident occurred on Sunday when a bus crashed off a bridge in North Hwanghae Province. Four North Koreans had also been killed in the accident.

China is North Korea's most important economic and diplomatic backer, although Beijing has been angered by Pyongyang's numerous missile and nuclear tests.

Chinese Ambassador to North Korea Li Jinjun said Chinese President Xi Jinping and the families of the crash victims would be notified of Kim's visit, while promising close cooperation with North Korean officials regarding the accident, the central news agency reported.

Kim also visited the hospital where the injured were being treated, the report said, adding that he "personally learned about the treatment of the wounded."

The North Korean leader was cited as saying the North's party and government would take follow-up measures to the accident "with utmost sincerity in a mind to alleviate the pain of the bereaved families even a bit."

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

Bilateral ties between the North and China showed signs of warming after Kim Jong Un made a surprise visit to Beijing in March, where he met with Xi Jinping and pledged to work towards denuclearising the Korean peninsula.