WASHINGTON -- The United States continues to urge South Korea to show "flexibility" in negotiations over sharing the cost of the American troop presence there, notwithstanding a recent agreement to have Seoul pay for Korean workers on US bases, a State Department official said Thursday.

The allies have for months tried to reach a new Special Measures Agreement stipulating how much South Korea will contribute to the cost of keeping 28,500 American troops stationed on the peninsula.

The negotiations have hit an impasse amid US demands for a significant increase in Seoul's share, but the two sides agreed earlier this week to let South Korea fund the labor costs for thousands of Korean workers who were furloughed by US Forces Korea in April due to the absence of a new deal.

Marc Knapper, deputy assistant secretary of state for Korea and Japan, said the development addressed the "human cost" of failing to renew the SMA, but also concerns about troop readiness.

"Ultimately the SMA is about ensuring that our alliance is capable and ready to deter and, if necessary, defend against North Korean aggression," he said during a virtual seminar hosted by the Center for Strategic and International Studies. "Having all these workers furloughed, we were concerned it would impact our readiness, impact our ability to honor our treaty commitments."

The recent agreement allows South Korea to pay for the labor costs through the end of the year, but it leaves the issue of reaching a new SMA unresolved.

Washington has asked Seoul to pay a total of $1.3 billion a year, a 50 percent increase from last year, while South Korea has insisted on a 13 percent increase.

"I think we believe we've been very flexible," Knapper said.

"Now we're urging the South Korean government to show the same flexibility."

Knapper also touched on last week's controversial transport of military hardware to a US missile defense base in South Korea.

Contrary to reports that it was aimed at reinforcing the unit, known as the Terminal High Altitude Area Defense system, Knapper said it was part of a routine upgrade.

"These systems require upgrades, just like your computer requires an upgrade every once in a while. So that was really it," he said.

Commenting on US President Donald Trump's invitation of South Korea to a Group of Seven summit in the fall, which President Moon Jae-in promptly accepted, the official emphasized that nothing has been finalized in terms of South Korea becoming a member of the group.

"It's a discussion still under way here in terms of what this will look like," he said, noting that as the host country the US has the prerogative to invite guests, as previous host countries have done.

"The only other thing I would say, though, is that expanding or changing the format or the membership permanently of the G7 requires consensus or unanimity among all the members," he added.

Meanwhile, amidst ongoing US-China tensions over civil liberties in Hong Kong, Knapper welcomed the South Korean foreign ministry's statement in support of the 1984 Sino-British Joint Declaration.

"The South Koreans came out with (what) I think was an unprecedented statement from them," Knapper said, citing the ministry's reference to the "one country, two systems" rubric ensuring Hong Kong's autonomy from China.

It was also "significant" that South Korea's statement came not long after it commemorated the 40th anniversary of the May 18 Democratization Movement in Gwangju, he said, "in recognition that South Korea enjoys the kind of democratic values that are under threat right now throughout the world and particularly in the region, and especially in Hong Kong." (Yonhap)