SEOUL (Reuters) – The United States opposes a plan by South and North Korea to set up a no-fly zone over their heavily fortified border, the latest sign of a rift between Seoul and its top ally, two sources familiar with the matter told Reuters. A man stands near binoculars as he tries to see North Korea’s propaganda village of Gijungdong at the Dora observatory near the demilitarised zone separating the two Koreas, in Paju, South Korea, April 24, 2018. REUTERS/Kim Hong-JiWashington and Seoul both publicly insist they are on the same page about dealing with Pyongyang. But behind the scenes, there are growing signs of disagreement as South and North Korea forge ahead with plans to defuse military tensions and rebuild economic ties. The military accord, sealed during last month’s summit in Pyongyang, is one of the most concrete agreements between the neighbors this year. But U.S. officials have raised concerns that it could undermine defense readiness and comes without substantial progress on denuclearization. The pact includes a halt in “all hostile acts,” a no-fly zone around the border and a gradual removal of landmines and guard posts within the Demilitarised Zone. U.S. Secretary of State Pompeo expressed “discontent” with the agreement during a phone call, South Korea’s Foreign Minister Kang Kyung-wha said last week in a rare disclosure of discord between the allies. The United States was not likely to openly protest against an inter-Korean initiative, Seoul officials said, but its deep involvement in sanctions enforcement and military operations give it leverage to delay or change the policy. The no-fly zone is a key sticking point for the U.S. because it would effectively prevent close air support drills, the sources said, adding that Pompeo raised the issue during the call with Kang. Both sources spoke on condition of anonymity because of the sensitivity of the matter. The zone, effective Nov. 1, will extend 40 kilometers north and south from the Military Demarcation Line in the East and 20 kilometers in the West for fixed-wing aircraft. The agreement also bars live-fire drills involving fixed-wing aircraft and air-to-ground guided weapons in the no-fly area. South Korea and the United States had held such drills regularly until halting joint exercises in June. There are different restrictions on helicopters, drones and balloons, with exemptions for commercial and non-military operations such as medical, disaster and agricultural uses. In close air support, airplanes provide firepower for troops who may be operating near enemy forces. Most fighter jets that U.S. forces operate in South Korea, such as the F-16, can play that role, one of the sources said. An official at Seoul’s foreign ministry said Pompeo had not been “fully briefed” when he complained about the inter-Korean deal, and called back that day to wish Kang good luck with the summit. The U.S. State Department declined to comment. Pentagon spokesman Lieutenant Colonel Christopher Logan declined to comment on the agreement but said the Department of Defense backs efforts to reduce military tensions. The department “remains in full support of our diplomats as they work to achieve the verified denuclearization of the DPRK as agreed to by Chairman Kim (Jong Un),” Logan said, referring to North Korea by its official name, the Democratic People’s Republic of Korea. A spokesman at South Korea’s defense ministry said Seoul cooperates closely with Washington and the U.S.-led United Nations Command (UNC). SIGNS OF DISCORD South Korea has held more than 50 rounds of talks this year with the UNC, which oversees affairs in the DMZ, to facilitate the inter-Korean commitments but has not yet secured its full support, according to Baek Seung-joo, a lawmaker of the opposition Liberty Korea Party. On Tuesday, North and South Korea held their first three-way talks with the UNC to discuss “practical” steps to facilitate the military pact, but did not announce any concrete measures, Seoul’s defense ministry said. “They’re not even close to an agreed definition of ‘hostile acts’”, said Baek, who served as a vice defense minister in 2013-15. “The agreement would make it impossible to carry out air cover and overall exercises, and it would hurt the alliance’s reconnaissance capabilities,” Baek said. But the UNC is taking a cautious approach so as not to kindle anti-U.S. sentiment among a public supportive of the two Koreas’ push for peace, Baek said. While maintaining that it remains in lockstep with Washington, the administration of South Korea’s Moon Jae-in has forged ahead with efforts to engage with the North, even as critics accused Moon of focusing on feel-good theatrics at the expense of progress. Kim vowed to work toward denuclearization during his unprecedented June summit with U.S. President Donald Trump. But Pyongyang’s actions have fallen short of U.S. demands for irreversible steps to scrap its arsenal, including a full disclosure of nuclear facilities and materials. “The alliance is being undercut for the sake of confidence-building with the North, while there is little progress on the nuclear issue, which is the root cause of the longstanding tensions,” Baek said. Reporting by Hyonhee Shin; Additional reporting by Idrees Ali and David Brunnstrom in WASHINGTON; Editing by Gerry DoyleOur Standards:The Thomson Reuters Trust Principles.