Almost all coal shipments to the Chinese city of Dandong, on the North Korean border, since February have been turned back, a source with knowledge of North Korean government operations in Dandong told CNN.
The ban followed strict, new sanctions imposed in November by the United Nations on North Korean coal exports, which China helped to draft and pass
Coal accounted for a third of all official North Korean exports in 2015,
making up a large part of its income. China is by far North Korea's largest trading partner.
Chinese Foreign Ministry Spokesman Lu Kang said turning back coal ships from North Korea was part of China "strictly carrying out our international obligations."
"This also isn't something new -- it's been our consistent policy," he said.
At a joint news conference with NATO Secretary General Jens Stoltenberg, Trump praised the move. "We have a very big problem in North Korea. And, as I said, I really think that China is going to try very hard, and has already started," Trump said.
"A lot of the coal boats have already been turned back -- you saw that yesterday and today -- they've been turned back," he said. "The vast amount of coal that comes out of North Korea going to China, they've turned back the boats. That's a big step, and they have many other steps that I know about."
Trump met with Chinese President Xi Jinping at Trump's Mar-a-Lago resort last week. The two then spoke on what the White House called a "very productive" phone call Tuesday night.
China 'proving it follows sanctions'
The new measures are designed to appease international critics who have argued China doesn't enforce UN sanctions against North Korea, CNN Beijing Correspondent Matt Rivers said.
"(It) gives them a clear rebuttal to the Trump administration's argument that China isn't doing its part," he said. "That could lead to greater leverage in future negotiations on other issues like trade."
But although China has shown some willingness to follow the US line, they would never push sanctions far enough to collapse Kim Jong Un's regime.
"It fears a united Korea under South Korean leadership, which could lead to US troops on China's border. A collapse could also lead to a refugee crisis," Rivers said.
Trump also signaled a willingness for the US to play a more confrontational role with North Korea, adding: "So we'll see what happens. It may be effective, it may not be effective. If it's not effective, we will be effective, I can promise you that."
Reuters, which first reported that the North Korean ships had turned back
to their home port of Nampo, citing its Thomson Reuters Eikon financial information and analytics platform, also reported that China is increasing coal orders from the United States.
No US coal was shipped to China between late 2014 and 2016, but 400,000 tons had been shipped there from the United States by late February, Reuters reported.
At Wednesday's news conference, Trump said Xi "wants to do the right thing."
"We had a very good bonding. I think we had a very good chemistry together. I think he wants to help us with North Korea," Trump said.
He also said the United States is willing to strike a trade deal with friendlier conditions for China if the country plays a role in deterring North Korea's nuclear program.
"I said, 'The way you're going to make a good trade deal is to help us with North Korea. Otherwise we're just going to go it alone,'" Trump said. "That'll be all right too -- but going it alone means going with lots of other nations."
North Korea reacted furiously to China's February ban, saying they were "dancing to the tune of the US"
and describing the cutting of imports as "inhumane."
Speaking to reporters Thursday, Chinese Customs spokesman Huang Songping confirmed that China stopped all North Korean coal imports after February 18.
Huang said that from January 1 until that cut-off date, China had imported 2.67 million tons of coal from North Korea, 51.6% less than the same quarter in 2016.
But despite the drop off in coal, the value of North Korea imports to China rose 18.4% in the first three months of 2017 compared to a year earlier.
He didn't provide a breakdown of trade, leaving it unclear how the gap was made up.