By David Brunnstrom

HANOI, June 2 (Reuters) - Vietnam's communist authorities have placed a prominent Buddhist dissident under house arrest for two years and arrested three other monks after they vowed a showdown on rights, a Buddhist support group said on Saturday.

Penelope Faulkner, spokeswoman for the Paris-based International Buddhist Information Bureau, told Reuters Thich Quang Do had telephoned her to say the detention order confining him to his pagoda in Ho Chi Minh City was effective from May 31.

Faulkner said three other monks from Ho Chi Minh City who had intended to accompany Do on a mission on June 7 to bring their detained 83-year-old patriarch Thich Huyen Quang to Ho Chi Minh City for medical treatment had been arrested.

Do, 73, is the deputy head of the outlawed Unified Buddhist Church of Vietnam, which Quang heads.

Faulkner said the three other monks, Thich Khong Tanh, Thich Quang Hue and Thich Tan An, from different pagodas in Ho Chi Minh City, were being held in jail in the southern city.

Police had been sent to many other pagodas in central and southern Vietnam to prevent monks joining Do on his mission, she said.

The report of the crackdown came two days before an historic trade agreement between the United States and Vietnam was expected to go to the U.S. Congress for ratification.

Vietnam's religious rights record has been cited as a factor that could complicate the ratification process.

The UBCV says Quang has been under under house arrest for 19 years in central Quang Ngai province, despite being officially released in November 1997. It says he is in poor health and needs urgent medical care.

PAGODA SURROUNDED

Faulkner quoted Do as saying that 30 police officers and communist officials came to his pagoda to inform him of the detention on Friday.

"He says there are now 100 security police outside the pagoda and 10 more inside," she said. "It's a formal order. He has been official sentenced to two years' house arrest."

Government officials could not be reached for comment.

Faulkner said she understood Do had been placed under "administrative detention," barring him from moving outside a set area without permission.

His detention followed the May 17 arrest of Nguyen Van Ly, a dissident Roman Catholic priest accused of spreading propaganda against the government.

Ly had called on the U.S. Congress not to ratify the trade pact until human rights conditions in Vietnam improved.

Faulkner said Do's vow to bring Quang back to Ho Chi Minh City for medical treatment had not been politically motivated.

"It has nothing to do with the trade agreement," she said. "He is concerned about the health of the Patriarch and is very worried he would not survive, that's why he has called for his release. This has nothing to do with international politics."

Hanoi insists its citizens have full religious freedom but has frequently been criticised by human rights groups for harassing clergy.

James Kelly, U.S. assistant secretary of state for Asian and Pacific affairs, was in Vietnam last month and raised the religious freedom issue with Foreign Minister Nguyen Dy Nien. He specifically referred to Ly.

He said afterwards that Ly's arrest would not help ratification of the trade pact between the former Vietnam War enemies.

A Democratic Senate aide in Washington told Reuters this week U.S. Trade Representative Robert Zoellick had indicated that the agreement, which was signed last July, would go to Congress for ratification next