House Foreign Affairs Committee Chairman Dante B. Fascell (D-Fla.) called on the Reagan administration yesterday "to take a more aggressive and assertive posture" in the Geneva arms negotiations and urged a high-priority effort to work out a comprehensive U.S.-Soviet ban on nuclear testing.

Fascell, in a House speech, announced that House leaders have given clearance for floor action later this month on a resolution backing test ban treaties. Last October, House leaders postponed action on the measure, which has 208 sponsors, after administration pleas that passage might complicate President Reagan's position at his summit meeting with Soviet leader Mikhail Gorbachev the following month.

Proposals to halt all nuclear weapons testing have been a key feature of Soviet policy for many months. Gorbachev's Jan. 15 arms offer extended until March 31 the self-imposed Soviet moratorium on nuclear tests.

Secretary of State George P. Shultz, testifying before Fascell's committee Wednesday, opposed a U.S. test ban in the absence of agreements on sharp reductions in nuclear weapons stockpiles. "As long as there are nuclear weapons there is a need to conduct tests" in order to ensure the safety of weapons stockpiles and to "carry out modernization" of nuclear weapons to match Soviet efforts, Shultz said.

Fascell, in his statement, said the objective of curbing nuclear weapons tests has been backed by all previous U.S. presidents since Dwight D. Eisenhower. "Without a forthcoming administration commitment to comprehensive test ban negotiations, the only propaganda winner is the Soviet Union and the only loser is the United States," Fascell said.

In addition to seeking new test ban negotiations, Fascell urged these administration actions in the Geneva arms negotiations:

*Proposal of a "comprehensive verification package" to test Moscow's expressed willingness to accept on-site inspection.

*Reaffirmation of mutual adherence to the SALT II treaty and "the longstanding interpretation" of the antiballistic missile (ABM) treaty.

*Use of the U.S.-Soviet Standing Consultative Committee to press U.S. concerns about Soviet compliance with arms treaties, and the rejection of Pentagon recommendations that U.S. treaty compliance be abandoned.

*Accelerated efforts to ban chemical weapons, include prohibition of production of new chemical weapons.