A Nicaraguan rebel fighter charged yesterday that the Nicaraguan government is using poison gas in battle, an allegation later denied by that government. U.S. State and Defense department officials said they had heard nothing about gas use in Nicaragua.

Osorno Coleman, a Miskito Indian and a commander of 200 "contras" fighting the leftist Sandinista government, spoke at a news conference called by Sen. Jeremiah Denton (R-Ala.). Coleman said that on Jan. 14 in a town called Howlover, a Soviet-made Mi24 assault helicopter used by government troops sprayed poison gas.

Coleman spoke in Spanish through an interpreter, Alejandro Salazar, another Nicaraguan, who lives in Florida. Salazar and Coleman were among several Nicaraguans who came to Washington to testify about alleged human rights abuses by Nicaragua's Sandinista government. The news conference in the Russell Senate Office Building was sponsored by the National Forum Foundation, a private educational group chaired by Denton.

Through his translator, Coleman described three days of combat. Sometimes not translating direct quotes, Salazar concluded: "After the helicopter had been flying over the trees, you could see how the leaves, in Coleman's words, were burned."

To questions, Coleman said, "That helicopter has special apparatus to spray poisonous gas. And as they withdrew, they were flying away as if spraying gas, and we believe that was what made the leaves dry."

Coleman's description of the gas and its effect -- except for the drying of the leaves -- was vague. "He did say," said Salazar, "that he saw the spraying -- but at a distance. And then, later, two people from the community told him it was poisonous gas."

Nicaraguan Minister Counselor Francisco Campbell said, in response to the poison gas charge, "I can assure you that the government of Nicaragua has not in the past and has no plans presently or in the future to use these types of weapons."

Speakers yesterday included two Nicaraguan Jews who alleged human rights violations by Sandinistas against the Jewish community and a 9-year-old girl who, through Salazar, stoically described how her mother and brothers were killed, she said, by Sandinistas.