The White House on Thursday expressed regret that Henry Kissinger was able to get more of an audience in Beijing than some sitting U.S. officials, after the former top diplomat held talks in China.

Kissinger - an architect of normalizing ties between Washington and Beijing in the 1970s as secretary of state and national security advisor in the administrations of Presidents Richard Nixon and Gerald Ford - was welcomed warmly as an “old friend” by Chinese President Xi Jinping on Thursday amid efforts by Beijing and Washington to mend frayed ties.

The White House said it was aware of the trip but that it was a private visit by a citizen.

As part of those meetings, Kissinger, 100, also met with China’s top diplomat Wang Yi and with defense minister Li Shangfu, who has declined direct talks with U.S. Defense Secretary Lloyd Austin.

General Li, appointed in March, remains sanctioned by the U.S. over his role in a 2017 weapons purchase from Russia’s largest arms exporter, Rosoboronexport. Chinese officials have repeatedly said they want those sanctions, imposed in 2018, dropped to facilitate discussions.

“It’s unfortunate that a private citizen can meet with the defense minister and have a communication and the United States can’t,” said White House National Security Council spokesperson John Kirby.

“That is something that we want to solve. This is why we continue to try to get the military lines of communication back open because when they’re not open and you have a time like this when tensions are high, miscalculations also, then the risk goes high.”

Kirby said that administration officials “look forward to hearing from Secretary Kissinger when he returns, to hear what he heard, what he learned, what he saw.”

Tensions between the world’s two largest economies have heightened over a range of issues, including the war in Ukraine, Taiwan and trade curbs.

Washington has tried to reestablish communication channels on these and other issues through recent high-profile diplomatic visits.

U.S. presidential envoy John Kerry concluded lengthy talks with Beijing on fighting climate change on Wednesday and current Secretary of State Antony Blinken went to Beijing last month.

President Joe Biden said last month that he wants to meet Xi in the coming months, with some officials hoping for face-to-face talks as soon as September’s Group of 20 summit in New Delhi or an Asia-Pacific Economic Cooperation gathering scheduled for November in San Francisco.