The United States has blocked a train project connecting North and South Korea amid deepening split between Washington and Pyongyang.

The two Koreas planned to begin a joint field study last week by sending a train from the South capital Seoul, across the length of North Korea all the way to Sinuiju on the Chinese border.

Their application, however, was denied by the US-led United Nations Command, which has controlled all movement through the demilitarized zone (DMZ) that bisects the peninsula, since the end of the Korean War some in 1953.

The blockade highlights a growing division between North Korea and the US, which has called for denuclearization as a prerequisite to any economic cooperation.

The US-led military body said in a statement that it requested “more fidelity on the details of the proposed visit.”

The project was proposed last month by South Korean President Moon Jae-in, for creating a “Northeast Asian railroad community” between the two Koreas, the United States and four parties, including China, Russia, Japan and Mongolia, according to the president’s office.