Grenadan Foreign Minister Unison Whiteman said yesterday that his government "has no plans" to grant military landing rights to any country at a new airport that President Reagan has said is a threat to U.S. security.

"To us, it is strictly a commercial facility" for tourism, Whiteman said during a meeting with Washington Post reporters and editors. "We have no agreements and no plans to use the airport for military purposes." The airport, with a 9,800-foot runway that is comparable in size to those in other Caribbean nations, is being built with Cuban as well some Western assistance. It is due to open next year.

In a speech two weeks ago, Reagan showed a reconnaissance photograph of the airport, which he said was part of the "Soviet-Cuban militarization of Grenada" and a "rapid buildup unrelated to any conceivable threat" against the small island nation of 110,000.

The Grenada airport has become a focal point for administration criticism of the leftist government that seized power four years ago from the repressive and widely disliked regime that had ruled since independence from Britain in 1974.

Denouncing Grenada's close relations with Cuba and the Soviet Union, the administration contends that they could use the island for military purposes that could disrupt Caribbean sea lanes or provide a landing spot for Cuban troops en route to Africa.

In addition to attempting to block Western European aid to Grenada and to exclude it from regional U.S. assistance programs, the administration has refused for the past two years to accept the credentials of a Grenadan ambassador here or to respond to diplomatic notes from Prime Minister Maurice Bishop's government.

Whiteman, who is in Washington for two days to meet with private groups and members of Congress, repeated charges by his government that it has information that the United States is backing a group of "mercenary forces" they expect to invade the island in a matter of "days or weeks." He declined to specify where the training was taking place or who was involved.