The U.S. government sped 12 helicopters and two cargo planes filled with 500 tents and 4,500 blankets to Colombia yesterday to aid victims of the volcano.

Government officials said they were awaiting word from two U.S. experts on the scene before deciding the next phase of relief efforts.

President Reagan, in a letter to Colombian President Belisario Betancur, said he was "stunned to learn of the devastation following the eruption of the volcano Nevado del Ruiz," and added that "in this hour of need . . . we want to work with your government and be of help."

Colombian Ambassador Rodrigo Lloreda met with officials of the Agency for International Development yesterday afternoon to relay what information he had on emergency needs in the disaster area, where as many as 20,000 are feared dead.

Lloreda said there is need for hospital supplies, medicine, and more tents to handle the 10,000 to 20,000 persons evacuating the area. "There is a real threat that the volcano might be reactivating in the near future," he said.

Jay F. Morris, deputy administrator of AID, which runs the Office of Foreign Disaster Assistance, said more supplies to aid Colombia are stockpiled in Panama and New Windsor, Md., and are prepared to be dispatched on orders from U.S. experts, the Pan American Health Organization and Colombian government experts on the scene.

In addition, six disaster relief specialists from the Dade County (Fla.) fire department and six from the Agriculture Department's interagency fire center at Boise, Idaho, are prepared to go to Colombia.

"We've got everthing and everybody ready and alert to react when details come in," said Morris, who estimated the U.S. outlay so far at $1 million, including $25,000 given to voluntary relief agencies in Colombia by U.S. Ambassador Charles Gillespie.

While at the State Department, Lloreda asked that Americans wishing to help victims of the eruption and the resulting floods and mud slides donate money rather than clothing or food. He said the Colombian government has opened an account with the National Bank of Washington to accept contributions.

Morris said mass evacuations are now impractical because more than a million people live in the region.

He said that after the volcano showed signs of activity Sept. 11, U.S. and United Nations experts prepared a hazard assessment for the volcano, a warning system and an evacuation plan. The plans were delivered to local officials Nov. 8, Morris said, too late to be used when the volcano erupted Nov. 13.