China's President Xi Jinping promised Cambodia hundreds of millions of dollars in aid and loans on Thursday during a state visit to the nation, which is one Beijing's staunchest regional allies.

Hundreds of students waving Cambodian and Chinese flags greeted Xi alongside officials at the airport before his motorcade sped into town for an audience with the royal family and later Prime Minister Hun Sen.

Cambodia has long been a strident supporter of Beijing's communist rulers, who have lavished the poor country with cash.

On Thursday the two leaders signed 31 agreements that saw China give Cambodia $178 million in aid for economic cooperation, a $59 million loan, plus an additional $15 million in military aid.

Xi also forgave a $89 million debt, the leaders said in a joint announcement after a signing ceremony.

China is Cambodia's top foreign investor and has given the country billions of dollars in grants and low-interest loans during Hun Sen's 31-year rule.

In July it offered Cambodia nearly $550 million in aid, days after the kingdom was accused of undermining regional unity by backing Beijing in disputes over the South China Sea.

In recent years Cambodia has become a thorn in the side for neighbouring nations hoping to present a unified front against China's island building in the contested waters.

Several Southeast Asian nations have competing claims to parts of the sea and many in the region want to keep pressure on China over its efforts to militarise the sea.

But Cambodia's unwavering support for China has scuppered regional efforts to jointly rebuke Beijing.

Ahead of Xi's trip, a leading Cambodian newspaper published an article signed by Xi that praised the Southeast Asian nation for coming to its defence over the sea row.

"When China acted to safeguard its territorial sovereignty and maritime interests and was devoted to resolving related disputes through peaceful negotiation, Cambodia did not hesitate to speak out to uphold justice," the Chinese president wrote in the article published by Cambodia's Rasmei Kampuchea newspaper.

He also described the two nations' friendship as "beaming with new vitality" and enjoying "deep political trust and win-win economic cooperation".

Large portraits of the Chinese leader and Cambodia's King Norodom Sihamoni were erected along the streets of the capital Phnom Penh ahead of the two-day visit, which is Xi's first there as president.

Hun Sen regularly praises Beijing's "no-strings-attached" aid, compared to help from the United States and European Union which is often accompanied by calls to address corruption and human rights abuses in his country.

"For Cambodia, China is the most important strategic and economic partner," said Vannarith Chheang, who chairs the Cambodian Institute for Strategic Studies, adding that Cambodia, in turn, is China's "most reliable friend in Southeast Asia".