Immigration officials, continuing to liberalize amnesty regulations, announced a policy change yesterday that will permit at least 100,000 additional illegal aliens to qualify for amnesty.

Speaking at a news conference here, Alan C. Nelson, director of the Immigration and Naturalization Service, also announced a policy change designed to help immigrant families stay together. In cases where parents qualify for amnesty, but their children do not, Nelson announced, the agency will not deport the children.

In cases where one parent is eligible for amnesty, but the other one isn't, INS officials will decide on a case-by-case basis whether to deport the children, Nelson said.

The change follows months of criticism from immigrant advocates, who argued that INS polices would split up thousands of families. Previously, anyone ineligible for amnesty faced deportation regardless of the status of other family members.

No estimates were available yesterday on how many families will be affected by the change.

But officials estimated that at least 100,000 people would benefit from the other decision announced yesterday. The immigration law enacted last November was designed to apply only to illegal immigrants who had lived in the United States since Jan. 1, 1982.

Under INS policy, an illegal immigrant who left the country and then returned legally, with a proper visa, had been considered ineligible for amnesty. The new policy makes those people eligible.

Amnesty is four-step process designed to lead to permanent residency and citizenship. As of this week almost 800,000 illegal aliens had applied for amnesty nationwide. INS expects 2 million to apply before the program ends in May.

The "family unity" provisions liberalize previous policy, but still leave in doubt cases where one spouse qualifies for amnesty but the other spouse does not, or where other immediate family members besides children are ineligible for amnesty.

While INS is prohibited from using information on an amnesty applicant's form to deport illegal family members, the agency can deport them if they come across those illegal aliens in other ways, such as in a work setting.

The American Immigration Lawyers Association has asked INS to permit all immediate relatives of an immigrant qualified for amnesty to remain and work in the United States.

"I understand that there's very little if any movement in that direction and that's one of the major factors that discourages large numbers of people from applying for legalization," said H. Ronald Klasko, an immigration lawyer in Philadelphia and president-elect of association.

Several lawyers say there are hundreds if not thousands of amnesty cases pending in which one spouse qualifies for amnesty, having lived in this country since Jan. 1, 1982, while the other spouse, who arrived here after that date, doesn't qualify.

"I think the agency is showboating to say 'we won't go against the children' when they will go against the spouses," said D.C. immigration lawyer Michael Maggio.

He said that one of the illegal immigrants detained by INS at the Quality Inn in Arlington last week faces possible deportation and separation from her husband, who has applied for amnesty.

In that case, INS fined the hotel $16,000 for employing 11 illegal aliens. It was one of the first actions against an employer since the sanctions provisions of the law took effect this summer.

The new immigration law calls for fining of employers of illegal aliens.

Yvonne Vega, executive director of Ayuda Inc., a private, nonprofit legal aid service for immigrants in the District, said her agency has about 60 cases in which one spouse qualifies and the other doesn't.

"I don't think the INS will be deporting children," she said. "They are more interested in the parents."

But Carmen Banegas, coordinator of the Centro Catolico of Maryland, an agency that is processing about 150 amnesty cases, said she knows of several families that will be helped by the new INS policy on minor children.

One of the cases involves three children, ages 10, 11 and 12, who arrived in this country from El Salvador in 1983 and do not qualify for amnesty. Their parents and an older brother, however, came to the United States in 1980 and do qualify, Banegas said.

"This is marvelous," she said. "They will be so happy {about the new policy} when I tell them."

At the news conference, Nelson said there had been a 30 percent decline this fiscal year in the number of illegal aliens apprehended at the Mexican border. He attributed the decrease to the employer sanctions.

In 1986, 1.6 million illegal immigrants were arrested trying to cross into the United States from Mexico, while in fiscal 1987 1.1 million were arrested, he said.