SEOUL, South Korea — South Korea gained a sanctions exemption that will allow it to take the first step in a plan to reconnect cross-border railways with the North, the latest step in efforts to improve relations between the rival nations despite slow progress in nuclear talks.

The U.N. Security Council approved a plan for the two Koreas to conduct a field survey that will involve the import of fuel and other materials to the North, the foreign ministry said Saturday.

The decision was limited to the survey. Further efforts to modernize North Korea’s railways and perform the work to reconnect the lines will be subject to additional exemptions, officials said.

The struggle for the exemption underscored concerns that differences between Seoul and Washington on the approach to the North could lead to a rift in the alliance forged in the 1950-53 Korean War.

The two Koreas also have blown up guard posts in a jointly patrolled part of the border, established a liaison office in a North Korean border town and conducted numerous cultural and sports exchange since the diplomatic process began in January, reversing rising tensions that had threatened to erupt in a nuclear war.

The United States has signed off on the steps taken thus far, and South Korea insists the allies are in lockstep and it will stay within the constraints of the sanctions regime.

But Secretary of State Mike Pompeo sounded a warning last week that the inter-Korean rapprochement should be dependent on advances in the U.S.-led effort to persuade the North to give up its nuclear weapons.

He noted the longtime allies established a working group this week to strengthen cooperation and “be sure that we don’t talk past each other.”

“We do want to make sure that peace on the peninsula and the denuclearization of North Korea aren’t lagging behind the increase in the amount of inter-relationship between the two Koreas,” he told reporters on Tuesday. “We view them as tandem, as moving forward together.”

The plan to reconnect the railways was first unveiled as part of an ambitious agreement reached between South Korean President Moon Jae-in and North Korean leader Kim Jong Un during their first summit in April.

Moon dangled it as an example of the economic rewards that could be reaped by the North if it abandons its nuclear program, which had prompted the Security Council to implement increasingly tighter sanctions in response to a series of missile and nuclear tests.

Seoul announced plans to hold a groundbreaking ceremony by the end of the year, initially saying the joint surveys wouldn’t violate sanctions before acknowledging that Washington had a different opinion.

The U.S.-led United Nations Command, which administers the southern side of the heavily fortified border, blocked passage of materials for the survey in August, citing procedural problems.

The railway project was a topic during the inaugural meeting of the working group chaired by the U.S. special envoy for North Korea Stephen Biegun and his South Korean counterpart Lee Do-hoon.

Unification Minister Cho Myoung-gyon said the survey could begin as early as this month, although he acknowledged the need for the North to denuclearize before more significant investment could be made in the railway project.

“What is left is the issue of coordinating with the North Korean side on schedules,” he told lawmakers Friday.

“We’ll go through the processes required under the sanctions regime,” he said. “I can say that the U.S. basically supports overall inter-Korean relations.”

Kim and President Donald Trump agreed to work toward the “complete denuclearization of the Korean Peninsula,” during their landmark summit on June 12, but talks have stalled over details.

North Korea has called for the easing of sanctions in its negotiations with the United States, saying it should be rewarded for steps already taken toward denuclearization including a testing moratorium on long-range and nuclear weapons.

The regime also has dismantled a missile engine testing facility and blown up its nuclear testing site, although U.S. officials and experts note the measures are likely reversible.

The Trump administration insists it will maintain economic pressure on Pyongyang until it sees more concrete measures, which have not been specified.

Stars and Stripes reporter Yoo Kyong Chang contributed to this report.

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