A former US national security advisor said Tuesday he is looking forward to a "big" contribution by South Korea in reference to ongoing negotiations between Seoul and Washington over the cost sharing for stationing the 28,500-strong US Forces Korea.

Robert O'Brien, who served as national security advisor for then President Donald Trump from 2019-2021, made the remarks as the allies have been engaging in negotiations over a new cost-sharing deal, called the Special Measures Agreement.

"South Korea can afford to pay for its defense," O'Brien told Yonhap News Agency in Milwaukee, Wisconsin, where the Republican National Convention is underway for a four-day schedule through Thursday.

"We look forward to seeing a big contribution for South Korea, which is already making by the way with its Navy and its Army and its military equipment ... It's doing great and I'm sure they'll have the money to do whatever defense it needs," he added.

O'Brien also highlighted that South Korea has become a "wealthy" nation.

"I'm looking forward to a good deal. South Korea (has) become a very wealthy country," he said. "I think the greatest economic success story of my lifetime is what's happened in South Korea."

Asked to comment on concerns that Trump could consider withdrawing American forces from South Korea if Seoul does not jack up its share of the cost for stationing USFK, O'Brien said, "It's all speculative."

O'Brien noted that Trump likes South Korea "very much."

"We had very good relations with South Korea when I was in office and when President Trump was there," he said.

"I think we got things toned down with North Korea. We (had) no nuclear tests, no ballistic missile tests. So he's very aware of what's happening in South Korea."

O'Brien also pointed out that Trump likes South Korean female golfers who dominated the Ladies Professional Golf Association.

"He's a big fan of South Korean golfers," he said.

O'Brien has been mentioned as a potential national security advisor for Trump should he return to the White House.

On Monday, Trump was officially nominated as the GOP standard-bearer for the Nov. 5 presidential election. (Yonhap)