BIEN HOA, Vietnam — U.S. Defense Secretary Jim Mattis got a firsthand look at the enduring costs of fighting the Vietnam War.

Mattis visited an air base north of Ho Chi Minh City that was heavily contaminated in the late 1960s and early 1970s by American forces through storage and spillage of the chemical defoliant Agent Orange.

Four years ago the U.S. pledged to clean and restore the parts of Bien Hoa (pronounced bee-yen WAH') air base that were contaminated.

U.S. Secretary of Defense Jim Mattis, right, is greeted by Vietnam's Air Force Deputy Commander Gen. Bui Anh Chung, second from right, as he visits Bien Hoa airbase, where the U.S. army stored the defoliant Agent Orange during the Vietnam War, in Bien Hoa city, outside Ho Chi Minh city, Vietnam, Wednesday, Oct. 17, 2018. (Kham/Pool Photo via AP)
U.S. Secretary of Defense Jim Mattis, right, is greeted by Vietnam's Air Force Deputy Commander Gen. Bui Anh Chung, second from right, as he visits Bien Hoa airbase, where the U.S. army stored the defoliant Agent Orange during the Vietnam War, in Bien Hoa city, outside Ho Chi Minh city, Vietnam, Wednesday, Oct. 17, 2018. (Kham/Pool Photo via AP)

The U.S. Agency for International Development soon will begin a soil restoration project at the base estimated to take several years and cost $390 million.

The officials said soil excavation at the base is scheduled to start next year, with contractors arriving at Bien Hoa by December.

After his visit to Vietnam, Mattis arrived in Singapore on Wednesday and he plans to meet on Thursday in Singapore with his Chinese counterpart amid escalating tensions over China’s purchase of Russian fighter planes and missiles as well as ongoing friction in the South China Sea.

The meeting planned for Thursday comes just weeks after their talks planned for Beijing fell apart.

Mattis and Chinese Defense Minister Wei Fenghe were in Singapore for a regional meeting of defense ministers. Mattis visited China in June, but since then a series of events have escalated tensions.

The assistant secretary of defense for Asian and Pacific Security Affairs, Randall Schriver, told reporters the Chinese had requested the Singapore meeting. In late September, China told the Pentagon that Wei would be unavailable to meet Mattis in Beijing, so that visit was canceled.

Schriver said the U.S. sees signs that the military-to-military relationship may be on the upswing.

"The fact that he's meeting with Minister Wei is some evidence that the Chinese are interested in keeping things normal and stable, as are we," Schriver said. "Our impression is that the (Chinese) military wants to keep things stable."

Schriver said the trigger for recent tensions between the Pentagon and the Chinese military was the Trump administration's decision in September to sanction the Chinese military for buying Russian fighter planes and missiles. That action was taken under the Countering America's Adversaries Through Sanctions Act passed by Congress in 2017.

China responded with strong criticism, followed in the military arena by a decision to cancel a planned visit to the Pentagon by the head of the Chinese navy and a confrontation in the South China Sea between a Chinese warship and a U.S. Navy destroyer, the USS Decatur.

"That may turn out to be a relatively short bump in the road," Schriver said, adding that Mattis is expected to convey to Wei U.S. interest in normal relations with the Chinese military.

More broadly, relations between the U.S. and China have deteriorated in recent months as escalating trade disputes and tariff hikes have been exacerbated by a newly announced U.S. military equipment sale to Taiwan.