The attacks against Cambodian footballer Keo Sokpheng after the national team lost to Myanmar at the Southeast Asian Games on Tuesday are the tactics of a slanderous opposition group, said Prime Minister Hun Sen.

Sokpheng, whose mother is Vietnamese, received racial abuse after missing the decisive penalty in a shootout against Myanmar that ended Cambodia’s hopes of a bronze medal.

“Being slanderous is the culture of a blind opposition group. To be clear, this group is the most deplorable and wants the country to sink. This group attacks anyone it wants to. It even accused King Father Norodom Sihanouk of colluding with the [Northern Vietnamese] Viet Cong.

“[This group] objects to Hun Sen’s government, accusing the Hun Sen government of selling land to Vietnam, of ceding land to Vietnam. That’s enough of that. It gets us nowhere.

“If it is to be like this, with one attacking the other, how can we work together? ‘When the water rises, the fish eats the ant; when the water recedes, the ant eats the fish’ . . . this is not my politics,” Hun Sen said at a graduation ceremony at the Koh Pich Convention Centre on Wednesday.

The prime minister said such politics dated from 2003 when an unnamed politician – likely Sam Rainsy, the “acting president” of the Supreme Court-dissolved Cambodia National Rescue party – said “Where there is [Rain]​ Sy, there is no [Hun] Sen; where there is Sen, there is no Sy”, meaning the two could not work together.

The prime minister said he did condone such divisive tactics.

Hun Sen said he had informed ambassadors and international officials of such politics, but they had failed to condemn the inciteful language of the opposition group.

Only Rhona Smith, the UN Special Rapporteur on the situation of human rights in Cambodia, had come forward to warn against such behaviour.

He said diplomats from certain countries had instead blamed the government for arresting members of the group without taking into account what they had done and what their “rebellious” leader had called for.

“Even with events on the sports field, [the opposition group] uses tactics to divide the nation, and then claims not to consider any Cambodian as an enemy. But in reality, they hold a grudge against us,” Hun Sen said.

He said because of their politics of slander, the opposition group had never won an election and would continue to lose.

Sok Touch, the president of the Royal Academy of Cambodia, said focusing on race was wrong because the majority of Cambodian people were of mixed heritage and not pure Khmer.