The FBI is investigating allegations that some congressmen have solicited and had sex with teen-age boys employed as congressional pages, according to sources familiar with the probe.

A Justice Department official confirmed yesterday that a 16-year-old former page told Justice and FBI officials that "he knows of claims by pages of sodomy and cocaine [use] . . . including with some congressmen."

The youth has provided investigators with the names of at least two and possibly as many as five congressmen, according to the sources.

"These are very serious allegations," the Justice Department official said. "We're very wary of it, but you have to look into it. You can't ignore it."

An FBI official said that the investigation is not an attempt to determine the private sexual preferences of congressmen or pages but is looking into possible criminal violations, including whether congressmen may have promised jobs in exchange for sexual favors or whether congressmen had sex with underaged boys.

"We've been interviewing ex-pages and pages all over the country," the FBI said.

About 100 congressional pages are sponsored each year by individual members of congress and hired to deliver messages on the floors of both houses, run errands and make deliveries to Capitol Hill offices for congressmen and senators.

The former page was sent to the Justice Department after he first made the allegations to staff employes of his congressional sponsor, the sources said.

Earlier this year, Capitol Hill police investigated the social activities of some pages, according to former pages and Capitol Hill sources.

One page whose activities helped touch off the police probe left the program before the investigation was completed, and two other pages were later asked to leave, according to Capitol Hill sources.

A U.S. grand jury here has been conducting a separate investigation into allegations that a drug ring used a network of Capitol Hill aides, including pages and tour guides, to distribute cocaine in congressional offices, according to sources familiar with that probe.