US Defense Secretary Lloyd Austin visited Cambodia on Tuesday in an effort to reset ties with the staunch China ally.
Washington's relationship with the Southeast Asian nation has been deteriorating for years, with China pouring in billions of dollars in infrastructure investments under Cambodia's former leader Hun Sen.
The United States has voiced concerns over a key Cambodian naval base that is being upgraded by Beijing, which it says could be used to boost the Asian giant's influence in the Gulf of Thailand.
En route from the Shangri-La Dialogue in Singapore, Austin landed in Phnom Penh Tuesday for the one-day visit to "explore opportunities to deepen our bilateral defense relationship", he wrote on social media platform X.
He first met Hun Sen -- who stepped down in August after a nearly four-decade rule over Cambodia -- and is also meeting Prime Minister Hun Manet, who took over from his father.
Cambodian foreign ministry spokesperson Chum Sounry told reporters last week that Austin's visit would be "another good chance to advance ties and relations of the two countries".
Both Austin and Hun Manet graduated from the United States Military Academy at West Point -— Austin in 1975 and Hun Manet in 1999.
Carl Thayer, emeritus professor of politics at Australia's University of New South Wales, said "US-Cambodia relations are at an inflection point after Hun Manet became prime minister".
He told AFP that Austin's visit to Cambodia "signifies that the two sides are willing to jettison some of their rigid policies restricting defence cooperation and engage in renewed dialogue to seek common ground".
Austin's visit comes just days after Cambodia and China wrapped up their largest annual military Golden Dragon exercises, involving several Chinese warships and hundreds of military personnel.
In early 2017 Cambodia scrapped a similar joint exercise with US forces.
In December, two Chinese warships made a first visit to the Ream Naval Base that Washington fears is intended for Chinese military use.
Cambodian officials have repeatedly denied that the base, near the port city of Sihanoukville, is for use by any foreign power.
Cambodian political analyst Ou Virak told AFP that the relaunch of joint exercises might be a point of discussion during Austin's visit.
The Pentagon chief would also want to convey a message to Beijing, "saying this region is too important and China won't have free rein", he added.
Washington has in the past voiced criticism over Cambodia's human rights record and crackdowns on political dissidents and critics.