Soviet and U.S. physicians signed an agreement today to monitor the 100,000 Soviets who face the greatest risk of contracting cancer from the Chernobyl nuclear accident, American bone marrow specialist Dr. Robert Gale announced here.

Gale, who helped direct the bone marrow operations on Chernobyl victims, said the long-term follow-up should include regular medical checkups and careful tracking of "cancer incidences in the Ukraine and surrounding area for a definite period of time." The agreement was signed by Gale and Dr. Andrei Vorobyov, a Soviet medical specialist.

Gale and Vorobyov appeared in a Moscow press conference following visits to the Chernobyl area and to Kiev, 80 miles to the south.

Gale provided new information on the health precautions imposed by Ukrainian officials after the April 26 incident, indicating that abortion counseling for potential radiation victims is proceeding and that additional measures to shore up the Chernobyl area water supply are being taken.

Soviet data have revealed that radiation levels in Kiev would be 15 to 30 times higher than normal in the first year after the incident, Gale said.

Following the explosion and fire at the No. 4 reactor at Chernobyl, 1,000 to 2,000 persons were checked at the site for radiation effects, Gale said, adding that it was necessary for 400 to 500 persons to receive follow-up treatment in Kiev and Moscow. He revealed that operations had been performed in Kiev immediately after the incident and that one of the victims had died in a Kiev hospital.

Gale said he expects the death toll, now 26 according to yesterday's Soviet reports, to rise only "slightly." "This figure may change slightly but will probably not change by greater than 10 percent," he said.

Five of the 13 most seriously injured radiation victims who required bone marrow transplants are still living, Gale said. Six others who received fetal liver injections have not survived.

Asked about rumors of abortions in Kiev conducted on potential radiation victims, Gale said he had been told that a commission set up to study the issue had advised doctors to counsel pregnant women on an individual basis. Gale said that no uniform recommendation about abortions -- which are legal and widespread in the Soviet Union -- had been made.

He also said that wells are being dug to provide an alternative water supply to the Dnieper River, in case irradiated soil from around the plant is washed into it.

Earlier this week Gale met with Ukrainian health officials in Kiev and took an extensive air tour of Chernobyl and Pripyat, the nearby small town.

Gale, the first American to tour the area since the incident, said the decontamination of Pripyat "is not going on as rapidly as one would like, for reasons that are not clear." But he said he was impressed with efforts to monitor water, milk and food in the Ukraine for radiation.

Radiation levels found in Kiev's water and milk supplies, Gale said, are "well below" those considered hazardous in the United States.

He said that of the 100,000 potential victims of long-term radiation, a "rather small" percentage eventually would contract cancer.

Vorobyov also explained that while most of those hospitalized for radiation were in the plant when the accident occurred, two cases came from the nearby town of Pripyat. "One of them was in the orchard," he explained; "the other was bicycling through the radiation area."