The incident took place in the northern Kampong Chhnang province, around 9o km north of the capital city of Phnom Penh, said Heng Ratana, director general of the Cambodian Mine Action Center.
The injured were taken to hospital in Phnom Penh by a helicopter. Two of them are in critical condition, he told Xinhua.
The experts were sent for a three-week course by the US Department of Defense to train Cambodians.
US Embassy spokesman John Simmons confirmed that four US personnel were injured but did not give any details.
Cambodia is one of the worst countries affected by mines after almost three decades of war and internal conflicts from the 1960s till 1998.
According to government records, since 1979 to November 2012, landmines have killed 19,661, injured 35,623 others and amputated 8,898.
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The country needs $30 million a year until 2020 to totally eliminate mines.