Religious groups that have helped to resettle nearly 75 percent of the 210,000 Southeast Asian refugees already admitted to the United States have declared their willingness to step up their efforts to assist the tens of thousands more who are still homeless.

Leaders of Roman Catholic, Protestant and Jewish agencies held a joint press conference here to urge both the United States and the United Nations to expedite the process by which refugees may be processed and brought to this country.

The American religious leaders appealed for public support to assure that there will be a continuing pool of sponsors for the incoming flood of new refugees. They also urged other countries to join in the humanitarian effort to resettle the refugees.

Participating in the press conference were the Rev. Paul McCleary, executive director of Church World Service, relief arm of the National Council of Churches; Dr. George F. Harkins, general secretary, Lutheran Council in the U.S.A.; the Rev. T. Grady Mangham Jr., director of World Relief Refugee Services of the National Association of Evangelicals; Rabbi Marc H. Tanenbaum, national interreligious affairs director of the American Jewish Committee; and Cardinal Terence Cooke, Roman Catholic archbishop of New York.

Mother Teresa of Calcutta, who was in the U.S. to establish new convents of her Missionary Sisters of Charity, made an unannounced appearance at the press conference and pledged the support of her order.

"I'm willing to give some of our sisters," Mother Teresa said. "Let us go into the sea and pick up the people . . ." who have fled their homeland by flimsy boats. She called on people of all faiths around the world to unite to solve the emergency.

The major Catholic and Protestant relief agencies that have handled the bulk of the resettlement say that the church response has been magnificent. "They are asking us, 'Where are the refugees? Why don't you send them to us?" one agency spokesman said.

Rabbi Tanenbaum said the central role being played by the U.S. religious community in refugee resettlement is a "great demonstration of religious faith in action." He described the current situation as the "greatest humanitarian crisis since the Nazi Holocaust."

McCleary said resettlement agencies will ask the government for extra funds, mainly for transportation, to bring in thousands of refugees now in temporary camps who are already cleared for admission to the U.S. He said current delays are due mainly to lack of transportation funds.

Cardinal Cooke said U.S. religious leaders are "seeking the cooperation of other religious bodies around the world, and we have some assurance of their support."

Nine voluntary resettlement agencies currently are cooperating with the U.S. State Department under the umbrellla of the American Council of Voluntary Agencies. Included are the U.S. Catholic Conference's Migration and Refugee Service; Lutheran Immigration and Refugee Service, International Rescue Committee, Church World Service, Hebrew Immigrant Aid Society, and the Evangelical Relief Agency. Catholics have resettled about 45 percent of all refugees admitted to the U.S.

In a joint statement, the religious leaders urged the following actions:

1. Doubling the U.S. quota for refugees admitted to long-term residence from 7,000 to 14,000 on an ongoing basis and to promote increased admissions of Indochinese refugees by other countries.

2. Establishment of transit camps or other temporary safe havens for those now drifting in the South China Sea.

3. Expediting movement, by United States military transport if necessary, of thousands of refugees already cleared by United States immigration authorities and waiting to join their sponsors in this country.