The chairman of the House ethics committee yesterday promised a prompt and thorough investigation of allegations that some congressmen have solicited and had sex with teen-age boys employed as congressional pages.

The announcement by Rep. Louis Stokes (D-Ohio) widened the scope of investigations by federal and local law enforcement agencies, including the FBI, into a range of alleged irregularities on Capitol Hill. These include allegations of sexual relations between pages and congressmen and charges that a cocaine and marijuana ring used pages and other congressional employes as couriers.

Stokes, chairman of the House Committee on Standards of Official Conduct, informally known as the ethics committee, described as "serious charges" reports of alleged "promised official acts" by congressmen "in return for sexual favors" from pages.

"Any charges that may be proved," he said, "will be reported to the House promptly for appropriate disciplinary actions."

Two weeks ago, the Justice Department's public integrity section initiated an FBI investigation into the matter after receiving allegations from one page that he had been solicited by a congressman and that several of his fellow pages had told him of sexual activities with congressmen.

Since that time, with the questioning of other individuals, additional allegations have been received that congressmen have solicited female pages for sex and that Capitol Hill staff members may have acted as go-betweens, approaching pages on behalf of members of Congress seeking sex, according to sources familiar with the inquiry.

No names of members of Congress have emerged publicly, and individuals close to the inquiry say fewer than six have been mentioned.

Justice Department spokesman John Russell has confirmed the FBI investigation is under way but has declined to provide details.

The allegations of illegal sexual activities can be traced initially to a few pages.

One teen-ager who served as a page supervisor--directing several other pages' work-- left Washington for home last winter.

The page had rented a two-bedroom apartment on Capitol Hill with another page starting in September 1981. After he departed, his landlady went through the apartment and reportedly found a billfold that had been reported missing. She arranged for it to be turned in to the Capitol police and at the same time described her former tenant as someone who had wild parties.

The Capitol police then began an investigation among the pages to determine "how the wallet got to that apartment," according to one House official. That inquiry, according to House sources, was expanded to cover the pages generally and turned up evidence of some drinking and use of drugs "but no reference to homosexual activity," the source said.

The former page, who by this time was home, was not interviewed then, according to this source who is familiar with the Capitol police inquiry.

Later, two other pages were sent home. According to other pages, they were told that one could not keep up his grades and the other was having difficulty living within his means. The other pages said, however, that the two pages who were sent home were friends of the page who had previously left and were involved in his partying.

The doorkeeper of the House, James T. Molloy, who is responsible for directing the activities of the roughly 70 House pages, visited at least one congressman sponsoring a page whose name came up in the investigation. At that time, Molloy assured the congressman that the page had done nothing wrong. He also bemoaned the lack of housing for all the pages, with the result that the males are left to find their own homes.

The pages, one source said after listening to Molloy at that time, "were problems just waiting to happen."

Yesterday, Molloy did not return a telephone call from The Post, but he was quoted by the Associated Press as saying he was "shocked" by news of the FBI investigation.

The first week in June, a page from Colorado who was sponsored by Rep. Patricia Schroeder (D-Colo.), was approached by a news organization and asked about the allegations that there were homosexual activities going on between other pages and members of Congress. The page reportedly told Schroeder and investigators that he never participated in such activities but was once solicited by a congressman.

This page, who had been a friend of the three who had left, thereafter went to Schroeder's aides and over a two-day period, June 10 and 11, told his story.

The aides took the story to Schroeder and it was decided to make an appointment for the page with the Justice Department's Public Integrity Section, which is charged with investigating allegations of official misconduct.

The father of the Colorado page was informed and he flew to Washington from Denver on June 13. The next day, the page, his father, and two Schroeder aides went to Justice. Accompanied only by his father, the page spent more than 90 minutes telling his story.

Thereafter, he and his father left Washington for Colorado.

During the next week, FBI agents interviewed other pages and House aides, according to sources close to the investigation.

Then last Friday, according to the Arkansas Gazette, a former page was interviewed by the FBI in Little Rock.

He told the Gazette Wednesday that the investigation was based on rumors and that "no one has any facts to back them up."

Pages deliver messages and material to members of Congress on the floors of both houses and make deliveries to offices on Capitol Hill. They also attend school in the mornings when Congress is out of session. For this work they receive about $700 a month out of which they must pay living expenses.

Most stay for one semester, although some stay longer and a few leave before their term is up.

They are appointed by members of both parties and some top House and Senate officials.

Over the years there have been problems with a few of the pages, most often stemming from the fact that they are teen-agers, dropped into this city with little supervision outside their school and working hours.