The pledge came from visiting Defence Minister Wei Fenghe, who arrived on Saturday for talks with his Cambodian counterpart and Prime Minister Hun Sen, said defence ministry spokesman General Chhum Socheat.

Wei, who leaves Wednesday, "promised to support, to train, and to help reform our military to be modernised like others", he said, adding that the two countries would next year hold their third series of joint military exercises known as "Golden Dragon".

The aid pledge comes at a time of worsening relations between the United States and Cambodia, which scrapped joint military exercises last year.

Hun Sen also accused the US of assisting in a plot by the opposition to topple his government, claims dismissed by the current ambassador as "absurd".

Hun Sen is looking to extend his 33-year grip on power in the vote next month after the main opposition party was dissolved last year and one of its leaders arrested.

Analysts say unwavering support from China in the form of loans and investment has allowed him to hammer away at threats to his rule without fear of consequences.

The US has withdrawn financial support for Cambodia's elections and this month sanctioned the head of Hun Sen's bodyguard unit for his alleged role in human rights abuses.

In a separate case, dozens of Cambodians who formerly worked at the US embassy in Phnom Penh protested outside it on Tuesday after being fired from their jobs in March, when allegations of sharing sexually explicit material and child pornography surfaced.

Carrying posters of Hun Sen and US President Donald Trump, they called on the embassy to provide evidence and asked for compensation.

"We were sacked from the jobs unjustly," said one former employee Chuon Im.

An embassy spokesman said it takes incidents involving child pornography and child exploitation "very seriously" but would not comment on internal personnel issues.