President Reagan yesterday appointed Robert Werner Duemling, a career diplomat and former ambassador to Suriname, to head the new office that will administer $27 million in humanitarian aid for antigovernment guerrillas in Nicaragua.

The White House announced last Friday that it had decided to distribute the aid for the counterrevolutionaries, known as contras, through a Nicaraguan Humanitarian Assistance Office that will be under the policy control of the secretary of State but separate from other parts of the State Department.

Reagan won a major victory last month when Congress, overcoming previous opposition in the Democratic-controlled House, voted to renew aid to the contras.

Congress specified that the funds can be used only for food, clothing and other humanitarian purposes, and it barred the Defense Department and the Central Intelligence Agency from any role in disbursing the funds.

Many congressional opponents of Reagan's Central America policies have voiced suspicion that the administration might stretch the definition of "humanitarian" to include equipment that the contras might use in their campaign to overthrow the Sandinista government.

In order to ease these suspicions and prevent the new office from becoming embroiled in controversy, administration sources said they wanted the program to be run by a career State Department official who would give the office a low profile.

Duemling, 56, has been a foreign service officer since 1957 and served as ambassador to Suriname from 1982 to 1984. Most recently, he has been serving with the State Department's management staff.

"We are mindful of congressional concerns and will consult with the Congress on assistance to be offered through the office . . . . "said State Department spokesman Bernard Kalb.