Australian Prime Minister Anthony Albanese has called for Beijing to be "transparent" about its plans in Cambodia, saying he's concerned by reports China is secretly building a naval facility in the country.
Cambodia on Tuesday again denied it would allow any Chinese military presence at the Ream Naval Base where it and China are beginning an expansion this week.
The expansion has prompted concern in Australia, the United States and elsewhere that it would be used by Beijing as a naval outpost on the Gulf of Thailand.
Asked about the report during on the final day of his trip to Indonesia, Albanese described it as "concerning".
"We're in regular contact with the Cambodian Government and we have been consistently assured that no foreign military will be granted exclusive access at Ream," he said.
"We've been aware of Beijing's activity at Ream for some time, and we encourage Beijing to be transparent about its intent and to ensure that its activities support regional security and stability."
Cambodia's chief government spokesman, Phay Siphan, described the expansion of the base as "cooperation between China and Cambodia".
He said the Chinese ambassador to Cambodia would preside over the groundbreaking on Wednesday along with Cambodia's defense minister and other senior military officials.
He denied, however, a report in the Washington Post newspaper citing an anonymous Chinese official that the facility on the northern side of the Cambodian base would be used in part by the Chinese military.
Siphan said it would be a violation of Cambodia's constitution to host a foreign military power and that there had been no change in the terms of his country's agreement with China on constructing the facility.
"I think that's a strong accusation," he said in a telephone interview.
He would not comment on the extent of the Chinese involvement and said the project involved constructing a facility for repairing ships.
Chinese foreign ministry spokesperson Zhao Lijian on Tuesday said the work would be a "renovation" of the base, which "aims to strengthen the Cambodian navy's capability to maintain maritime territorial integrity and combat maritime crime".
Ream faces the Gulf of Thailand, adjacent to the South China Sea, where China has aggressively asserted its claim to virtually the entire strategic waterway.
The US refuses to recognise China's sweeping claim and routinely conducts military manoeuvres there to reinforce that they are international waters.
A Chinese base in Cambodia could become a chokepoint in the Gulf of Thailand close to the strategically important Malacca Strait.
State Department spokesman Ned Price said Monday allegations China intended to establish a military presence at the Ream base were "consistent with credible reporting we've seen from the PRC (People's Republic of China) that the PRC is engaged in a significant ongoing construction project at Ream Naval Base."
"As we've said, an exclusive PRC military presence at Ream could threaten Cambodia's autonomy and undermine regional security as well," he said.
China recently signed a security deal with the Solomon Islands that Australia, the US and others worry could lead to a military presence there, and Price noted that Beijing has also reached out to a number of other South Pacific islands.
"We have seen the PRC attempt to put forward a series of shadowy, opaque deals that they would like to see signed in the dead of night with no input or transparency," he said. "This has been a pattern on the part of the PRC."
The US itself has more foreign military bases than any other country, including multiple facilities in the Asia-Pacific region.
At a regular foreign ministry briefing, Zhao accused the US of "bullying" Cambodia and ignoring its denials Ream would be used for Chinese military purposes, while noting Washington's own network of bases around the world.
"China and Cambodia are comprehensive strategic partners, and our cooperation in various fields is open, transparent, reasonable and legitimate, which benefits the two countries," he said.
China so far operates just one acknowledged foreign military base, in the impoverished but strategically important Horn of Africa nation of Djibouti.
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Many believe China's People's Liberation Army is busy establishing an overseas military network, even if they don't use the term "base".
Cambodia's authoritarian leader, Hun Sen, has long cultivated relations with China, and reportedly signed a secret agreement in 2019 allowing the Chinese to establish a base at Ream.
Though Hun Sen has strongly denied Cambodia would allow China to set up a military outpost at Ream, China has already been dredging the harbour to allow larger ships to dock, and is building new infrastructure to replace a US-built naval tactical headquarters.
Hun Sen suggested last month the water would still be too shallow for any warships to dock, and reiterated that it would be a violation of Cambodia's constitution to host a Chinese military facility.
"Why would we need foreign forces, for what?" he said.
"What Cambodia really needs at the moment is foreign investment, not foreign forces."
The Asia Maritime Transparency Initiative, established by the Washington-based Centre for Strategic and International Studies think tank, posted satellite images in earlier this year of two "clamshell" dredgers at work in the harbour.
It said while the extent of the dredging was unknown, it could mark a "significant upgrade" to the base.
"The shallow waters around Ream mean it is currently only able to host small patrol vessels," the organisation said.
"A deep-water port would make it far more useful to both the Cambodian and Chinese navies."
Newer images from April analysed by The Associated Press show the dredgers still on site and at work.