The appeal was issued on Tuesday by the UN special rapporteur on human rights in Myanmar, Paulo Sergio Pinheiro, as well as UN investigators on torture, adequate housing, food, health and minority issues.
They said Myanmar's army has been driving thousands of ethnic Karen out of villages close to the government's new jungle capital, near the town of Pyinmana, in what appears to be the biggest offensive against the group in 10 years.
"We call on the government of Myanmar to take urgent measures to end the counter-insurgency military operations targeting civilians in Northern Karen and Eastern Pegu areas, which have led to the forcible eviction and displacement of thousands of ethnic minority villagers," the investigators said.
Burmese soldiers have acted with "excessive use of force and firearms", according to the joint statement.
Myanmar's generals admit they
are targeting the Karen
"Other reports from various sources corroborate very serious allegations of unlawful killings, torture, rape and forced labour," it said, without elaborating.
Myanmar's generals admitted this weekend they were targeting the Karen, a mainly Christian ethnic minority of 7 million people, or just over 10% of Myanmar's population.
But Brigadier-General Kyaw Hsan, the information minister, said the offensive was meant to quell anti-government military action by the Karen National Liberation Army (KNLA), which has waged a 50-year civil war.
Karen civilians were displaced without access to food, education, housing and health services, according to the UN investigators, who also said that those whose homes had been demolished had not been offered alternative housing or compensation.
Myanmar accuses Karen rebels
of anti-government activity
Ibrahim Gambari, the UN undersecretary-general for political affairs, begins a three-day visit to Myanmar on Thursday, the first by a UN official in more than two years.
Kofi Annan, the secretary-general, said Gambari "will convey a clear message that Myanmar's prospects for improved relations with the international community will depend on tangible progress in restoring democratic freedoms and full respect for human rights".