Two sources familiar with the matter told Reuters news agency on Thursday that US officials have raised concerns that the no-fly zone could undermine defence readiness and comes without substantial progress on denuclearisation.
Washington and Seoul both publicly insist they are on the same page about dealing with Pyongyang.
However, 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 two Asian neighbours this year.
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.
US 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 US 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 US 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 to Reuters on condition of anonymity because of the sensitivity of the matter.
The zone, effective November 1, will extend 40km north and south from the Military Demarcation Line in the east and 20km 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 US 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, aeroplanes provide firepower for troops who may be operating near enemy forces. Most fighter jets that US 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 US State Department declined to comment.
Pentagon spokesman Lieutenant Colonel Christopher Logan declined to comment on the agreement but said the Department of Defence backs efforts to reduce military tensions.
The department “remains in full support of our diplomats as they work to achieve the verified denuclearisation 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 defence ministry said Seoul cooperates closely with Washington and the US-led United Nations Command (UNC).
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 legislator 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 defence ministry said.
“They’re not even close to an agreed definition of ‘hostile acts'”, said Baek, who served as a vice defence 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-US 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 towards denuclearisation during his unprecedented June summit with US President Donald Trump.
But Pyongyang’s actions have fallen short of US 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 long-standing tensions,” Baek said.