-NATO military leaders said here today that the alliance should deploy both ballistic and cruise missiles on the ground in Europe to combat the Soviet mobile SS 20 missiles, according to partcipants in the Nuclear Planning Group meeting.

An official who participated in the discussions on how best to modernize NATO's nuclear weapons said there was "general acceptance" among military officers that both types of land missiles were needed to make it harder for the Soviets to counter them.

The U.S. Army's Pershing II land missile, with a maximum range of about 1,500 miles but less if it carries a big nuclear warhead, is the favored weapon because it could be deployed in a hurry.

Although NATO military leaders are considering building a new medium-range ballistic missile that could hit the Soviet Union from Western Europe, this weapon is still an idea on paper and thus several years away from deployment.

A land-based cruise missile-essentially a drone airplane carrying a nuclear bomb-could be deployed as early as 1983 because the United States is already testing an air-launched version of the weapon.

Medium-range missiles, which fly up to about 3,000 miles, are not covered by the pending strategic arms limitation treaty (SALT II). Only strategic nuclear weapons with ranges up to 6,000 miles are covered by SALT II.

Conference officials, while trying to project a sense of urgency about the need to modernize NATO's theater nuclear weapons, said that no decisions on that isue are expected to be made until at least December when another NATO group, the Nuclear Planning Committee, meets in Brussels.

Some high Carter administration officials are conceding that they bungled the neutron warhead decision by stirring up the opposition before they presented their case for the weapon in a cohesive way.

"We don't want to follow the road off the cliff again," said one official in vowing that the case for deploying a nuclear missile in West Germany that could hit the Soviet Union will be presented much more carefully.

The NATO defense ministers in their closed discussions today, conference officials said afterward, did not debate the neutron warhead all over again, but instead focused on medium-range missiles that would provide more nuclear punch.

The NATO defense ministers and the military officers who accompanied them apparently looked rather calmly on the new Soviet SS21 battlefield nuclear missile that has recently been deployed with ground units.

One official said that Soviets are following the U.S. lead by modernizing-replacing the old Frog battlefield missile with the SS21, just as the United States replaced the Honest John rocket with the Lance battlefield missile.

The Nuclear Planning Group winds up its discussions here Wednesday. The group is comprised of defense ministers from Britain, Canada, Italy, the Netherlands, Turkey, the United States and West Germany.