China has resumed supplies of fertilizer and food to North Korea and doubled oil shipments after North Korean leader Kim Jong-un's third visit to China in June.

The move has exacerbated fears that China can effectively sabotage international sanctions against the North and thus scupper denuclearization efforts.

Though the exact figure is not known, China sent an estimated 200,000 to 300,000 tons of fertilizer to the North, according to sources in China.

In 2013, China supplied the North with 200,000 tons of fertilizer. Kwon Tae-jin of the GS&J Institute, an agricultural think tank in Seoul, said, "There are different ways of calculating harvest rates for different types of fertilizer and soil. But in case of urea fertilizer, it's possible to produce two tons of food more for each ton of fertilizer."

"Beijing's policy is to continue exchange and cooperation with Pyongyang as long as they don't violate UN sanctions," one source said. "It seems China sent the latest supplies of fertilizer on the pretext of humanitarian relief."

China also dramatically increased oil shipments to the North. A source in Beijing said it nearly doubled crude oil supplies to the North through pipelines from Dandong since Kim's recent visits.

"Some 30,000 to 40,000 tons of oil is enough in the summer to maintain the lowest possible flow of oil in the pipelines to ensure that they don't clog, and about 80,000 tons in winter," the source added. "Though it's summer now China has recently increased flow to the winter level."

The UN Security Council has limited crude oil shipments to North Korea to 4 million barrels or 560,000 tons a year. If China sends 80,000 tons of oil to the North every month, that already brings the amount to 960,000 tons a year.

North Korea in turn ships minerals and medicinal herbs to China, and a lot of food is smuggled in from China as border security loosened since Kim's China visits. Food prices in the North are stabilizing.

In June, large trucks carrying North Korean iron ore were spotted crossing the bridge from Namyang, North Hamgyong Province to Tumen in the Chinese province of Jilin, even though exports of iron ore are banned by the UNSC.

But it is still difficult for China and North Korea to resume larger-scale cooperation projects.

"Despite its promise of large-scale aid to Kim, it's not easy for China to continue because that would violate UN sanctions," said Cho Bong-hyun at the IBK Economic Research Institute. "It seems that China is giving the North just enough aid to get some breathing space."

