Seoul, South Korea (CNN)Three North Korean cargo planes flew to China and back on Monday, as the country battles an fast-spreading outbreak of Covid-19, according to a South Korean government official with knowledge of the matter.

The planes traveled to Taoxian International Airport in Shenyang, in China’s northeast Liaoning province, the official said. It’s unknown what the planes were carrying, but the rare trip came after China pledged to help North Korea with its Covid outbreak, which experts have warned could cause a major humanitarian crisis in the isolated and impoverished nation.

North Korea officially confirmed its first ever Covid cases last week. It had not previously acknowledged any cases, and has kept its borders tightly shut since January 2020. Since May 12, North Korea has reported nearly two million “fever” cases, with state media calling it a “major national emergency” and authorities scrambling to respond. All cities have been placed under lockdown, and North Korean leader Kim Jong Un has mobilized the military to help secure the supply of medicine in the capital Pyongyang.

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