Defense Secretary James Mattis says he thinks the world will exert “increased pressure” on North Korea in the coming months following recent maritime incidents in waters near China and the controversial development of its nuclear weapons program.

South Korea said Friday that it inspected a Hong Kong-flagged ship for allegedly violating U.N. Security Council sanctions by delivering oil to a North Korean vessel in October.

"I think you will see increased pressure,” Mattis told Fox News on Friday. “What form that pressure takes in terms of physical operations is something that will be determined by the cognizant governments. Obviously if a government finds there is a ship in their port conducting trade that was forbidden under the U.N. Security Council resolution then they have an obligation and so far we’ve seen nations take that obligation seriously.”

Mattis also told Fox News that “nothing impresses me” when asked about the development of North Korea’s missile program.

An official from Seoul's Foreign Ministry told the Associated Press Friday that the Hong Kong-flagged ship in the October incident, the Lighthouse Winmore, is believed to have transferred about 600 tons of refined petroleum products to the North Korean ship in international waters on Oct. 19 after leaving the South Korean port of Yeosu.

The official says South Korean authorities boarded the ship and interviewed crew members after they returned to Yeosu on Oct. 24.

Fox News also reported Thursday that U.S. spy satellites reportedly have captured photos of Chinese ships illegally selling oil to North Korean boats some 30 times since October.

Satellite images released by the U.S. Department of Treasury appeared to show vessels from both countries illegally trading oil in the West Sea, The Chosun Ilbo reported Tuesday, citing South Korean government sources.

“Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea. There will never be a friendly solution to the North Korea problem if this continues to happen!” President Donald Trump tweeted Thursday while on a holiday break in Florida.

The U.S. Treasury in November sanctioned North Korea’s Maritime Administration and its transport ministry, in addition to six North Korean shipping and trading companies and 20 of their vessels, in an effort to block the rogue regime’s transportation networks.

North Korea was barred in September by the United Nations Security Council from importing natural gas and had its crude oil imports capped in response to Kim Jong Un’s nuclear missile program.

While Russia exports some oil to North Korea, China is the main source of oil for the rogue nation, according to Reuters. However, the country exported no oil products to the North during the month of November. It was reportedly the second consecutive month China didn't export diesel or gasoline to North Korea.

Chinese foreign ministry spokeswoman Hua Chunying said “the Chinese government has been completely and strictly enforcing Security Council resolutions” aimed at discouraging North Korea from developing nuclear and missile technology.

Fox News’ Lucas Tomlinson, Nicole Darrah and the Associated Press contributed to this report.