PHNOM PENH -- The United Nations said on Sunday Vietnam and Cambodia were undermining an agreement to voluntarily repatriate asylum seekers fleeing Vietnam's restive central highlands.

The two Southeast Asian governments on Friday announced an April 30 deadline for returning ethnic minorities to Vietnam during a controversial trip to a UN refugee camp in Cambodia on Friday. The UN High Commissioner for Refugees (UNHCR) said the deadline jeopardized the "voluntariness" of returns.

Police in Cambodia used electric shock batons to subdue several asylum seekers who vociferously objected to going back to Vietnam during the visit, rights activists and witnesses said.

More than 1,000 Vietnamese minorities, most of them members of Christian hilltribes who supported the United States during the Vietnam war, are housed in two UN facilities in Cambodia, Reuters reported.

They fled the central highlands after Hanoi sent troops last year to quell protests over land rights and religious freedoms.

Hanoi, Phnom Penh and the UNHCR agreed last month to begin voluntarily repatriating them, but the deadline set during a controversial visit by Vietnamese officials to one camp on Friday has upset both the UN and the United States.

"A tripartite agreement with Vietnam and Cambodia on the voluntary return of Vietnamese Montagnards (hilltribe peoples) was being seriously eroded by the actions of the two governments," the UNHCR said in a statement from its headquarters in Geneva.

"The refugee agency is seeking urgent clarification from Cambodian and Vietnamese authorities."

UNHCR is also unhappy it had to withdraw a team of monitors from Vietnam's central highlands on Thursday, after Hanoi refused access to villages where asylum seekers planned to return.

The pullout followed the first UN voluntary repatriation of 15 asylum seekers on Tuesday.

The U.S. government and Human Rights groups have blasted the repatriations, saying adequate safeguards were not in place to ensure their safety.

Several of the 15 people who returned to the highlands province of Kontum told Reuters they were still worried.

The UNHCR said in the statement that it opposed Friday's surprise visit of the high-level Vietnamese delegation to its Cambodia camp, home to more than 500 of the asylum seekers.

"In general, UNHCR opposes visits to refugee camps by officials from countries they have fled," the UNHCR said.

During that visit, UN staff were forced to intervene when a policeman struck a number of camp inmates with an electric shock baton.

The United States, which has resettled some of those who fled Vietnam, has also expressed opposition to the return deadline and said asylum in America was still an option.