BEIJING (CNN) -- The docking of a North Korean ship at a port in northern China has raised questions over a Chinese ban on coal imports from the nuclear-armed state.
The Jin Hung 9 docked at a port in Tangshan city in northern Hebei province, near Beijing, on Thursday morning.
According to a daily docking plan published on Jintang port's website, the North Korean ship was loaded with anthracite coal. The website listed a nearly six-hour unloading time for the ship.
What the Jin Hung 9 unloaded is not clear.
Speaking Friday, Chinese Ministry of Foreign Affairs spokesman Lu Kang said the country's policy of not accepting coal imports from North Korea had not changed.
"There have been some reports that some coal ships have tied up in port. You are aware if these ships continue to remain out at sea, away from port, we need to make some humanitarian consideration for their crews," Lu said.
He added that "nothing that violates the implementation" of sanctions had occurred.
At least four other North Korean ships were listed as having docked at the port, but it is unclear what their cargo was.
China said in February it was halting all coal imports from North Korea for 2017 in accordance with a UN Security Council resolution passed last year.
"Imports of coal produced in North Korea -- including shipments already declared to the customs but yet to be released -- will be suspended for the remainder of this year," China's Ministry of Commerce said in a statement.
Coal is North Korea's main export and an important source of foreign currencies for its fragile economy. Most of North Korea's exported coal is shipped to China, its only major ally on the global stage.
The link between Beijing and Pyongyang has been frayed by North Korea's repeated missile tests, and increased pressure from Washington on China to act to prevent further nuclear tests by Pyongyang.
North Korea has conducted five nuclear tests since the early 2000s, all of which have been roundly condemned by China.
On Thursday, a day after North Korea's Vice Foreign Minister said Pyongyang would test missiles weekly and use nuclear weapons if threatened, Chinese Foreign Ministry spokesman Lu said Beijing was "gravely concerned" about North Korea's recent nuclear and missile activities.
Lu also praised recent US statements about "using whatever peaceful means possible to resolve the (Korean) Peninsula nuclear issue."
Tensions have been on a knife edge on the peninsula amid tit-for-tat saber rattling from Pyongyang and Washington and a major military parade in North Korea, during which several potential new intercontinental ballistic missiles (ICBMs) were unveiled, and a failed missile launch.
During a recent visit to Japan, US Vice President Mike Pence called North Korea the "most dangerous and urgent threat" to the region and vowed any North Korean attack on US forces or their allies would be met with "overwhelming" force.
According to US defense official, Chinese air force land-attack, cruise-missile-capable bombers were put on "high alert" earlier this week as part of what is believed to be a Chinese effort to "reduce the time to react to a North Korea contingency."
The official said the US also has seen an extraordinary number of Chinese military aircraft being brought up to full readiness through intensified maintenance.
On Friday, South Korea said it was closely watching North Korea in days ahead because it could "carry out a provocation at any time" around the April 25 anniversary of the founding of the Korean People's Army.
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