KAOHSIUNG, Taiwan — The two ships met in daylight in the middle of the East China Sea. One was an 11,253-ton oil tanker, the Lighthouse Winmore, supposedly heading to Taiwan. The other, an aging freighter, was emblazoned with the red, white and blue flag of North Korea.

In an illicit high-seas exchange captured in photographs taken by an American spy plane, the Lighthouse Winmore offloaded what officials later said was 600 tons of oil to the North Korean vessel in violation of United Nations sanctions.

With those sanctions constricting its trade, including the import of refined petroleum, North Korea has increasingly turned to illegal clandestine shipments to acquire the fuel it needs, according to diplomatic officials and documents obtained by The New York Times.

Trafficking on the high seas has become what these officials regard as a pernicious subversion of the sanctions. Though the frequency of smuggling is difficult to estimate, they fear it is undermining efforts to thwart the nuclear weapons ambitions of Kim Jong-un, the North Korean leader, through economic pressure. The trafficking has also strained relations between the United States and North Korea’s two largest trading partners, China and Russia.