SEOUL (Reuters) - South Korea has sent 1 billion won ($893,239.96) worth of sanctioned material to North Korea, customs data showed on Friday, raising concern ties with the United States could be damaged as the South pursues cooperation with the North.

South Korean soldiers stand guard at the truce village of Panmunjom inside the demilitarized zone (DMZ) separating the two Koreas, South Korea, April 18, 2018. Picture taken on April 18, 2018. REUTERS/Kim Hong-Ji

The United States is pressing North Korea to give up its nuclear weapons program, calling for stringent implementation of tough U.S. and U.N. sanctions.

But at the same time, its ally, South Korea, is trying to increase cooperation with its old rival.

South Korea transferred about 113 tonnes of materials and equipment including steel, copper, nickel and water heaters to North Korea in June and July, according to customs data released by opposition lawmaker Cheong Yang-seog.

The material and equipment identified in the data are banned from being exported to North Korea under U.N. Security Council Resolution 2397, Cheong’s office said.

The Korea Customs Service declined to comment on the data, directing inquiries to another ministry that handles inter-Korean relations. It also declined to comment.

A South Korean foreign ministry official also declined to comment on the customs data, but said materials and equipment are being sent for a liaison office that South Korea is building just over the border in North Korea, as part of efforts championed by the South’s President Moon Jae-in to improve ties.

“All the material, equipment and electricity are for the office’s operation and to ensure the convenience of our personnel,” said the ministry official.

“It does not give any economic gain to North Korea, so we are concluding that it does not damage the objective of the sanctions.”

Moon is committed to securing diplomatic progress with North Korea, in part by opening this year the office in Kaesong, on the North Korea side of the border.

Cheong was not available for comment but he has been raising objections to the lack of information on the amount of money the government has been spending on the liaison office.

He and other opposition politicians have raised concern that the transfer of material for the office could violate U.N. and U.S. sanctions against North Korea.

Last month, South Korea’s customs agency said three South Korean firms imported coal from North Korea disguised as Russian products in violation of U.N. resolutions.

Shin Beom-chul, senior fellow at Asan Institute for Policy Studies in Seoul, said such sanctions busting, together with South Korea’s push to expand ties with the North, could exacerbate friction with the United States on how to handle North Korea.

“If we go on like this the gap of disagreement will widen, and can become a more serious issue,” Shin said.

North Korean leader Kim Jong Un agreed in broad terms to work toward denuclearization of the Korean peninsula in a historic summit with U.S. President Donald Trump in June.

But there has been no indication that North Korea had stopped its nuclear activities, the International Atomic Energy Agency (IAEA) said in a report on Monday.

U.S. Secretary of State Mike Pompeo is set to visit North Korea next week.