Story highlights China banned all import of coal from North Korea in February

Chinese Ministry of Foreign Affairs spokesman says policy hasn't changed

Beijing (CNN) The docking of six North Korean ships at a port in northern China has raised questions over a Chinese ban on coal imports from the nuclear-armed state.

The ships docked Thursday and Friday at a port in the city of Tangshan in northern Hebei province, near Beijing.

All six North Korean ships were loaded with anthracite coal, according to a daily docking plan published on the port of Jintang's website. Unloading times of up to six hours were listed for the ships.

Three of the ships -- the Jin Hung 9, the Woory Star and the Kum Gang San 2 -- had been due to dock since February, when China said it was halting all coal imports from North Korea for 2017 in accordance with a UN Security Council resolution passed last year.

It's unclear what the ships unloaded. The three have since left the port.

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