The U.S. cited a quantity of diesel fuel that would be carried on a South Korean train running on North Korean railway tracks to block a plan by the two Koreas to survey a cross-border railway. Diesel exports to North Korea are banned under UN Security Council sanctions.

Vice Unification Minister Chun Hae-sung asked U.S. Forces Korea Commander Vincent Brooks, who is also the head of the UN Command which oversees the armistice, to let the survey go ahead, but to no avail.

"We can't disclose in detail the items to be shipped to and from the North," a Unification Ministry spokesman told reporters in a press briefing last week.

But a diplomatic source said, "They'll have to use an electric locomotive and a diesel engine to power the cross-border railway survey, which could violate UN and U.S. sanctions."

The intervention came as denuclearization talks between the U.S. and North Korea reached a fresh impasse.

The two Koreas agreed at their summit in April to see how the railway from Seoul to Sinuiju on the North Korea-China border can be reconnected. It is currently impossible for trains to run across the border because South and North Korean trains work on different voltages, so diesel locomotives would have to be used instead.

A UNSC resolution from last December, bans all member states from "the direct or indirect supply, sale or transfer to [North Korea], through their territories or by their nationals, or using their flag vessels, aircraft, pipelines, rail lines, or vehicles... of all refined petroleum products" over an annual limit of 500,000 barrels.

The U.S. believes that the North has already secured more than 500,000 barrels of refined oil this year through illegal ship-to-ship transfers.

