MADRID, NOV. 6 -- The United States and Spain resolved today to continue negotiating on reductions in U.S. military forces here despite Spain's intention to renounce the 34-year-old treaty allowing U.S. bases on Spanish soil.

The chief Spanish negotiator, Maximo Cajal, and the head of the U.S. team, Ambassador Reginald Bartholomew, said after two days of unsuccessful talks that the two sides will meet again in mid-December after Spain formally notifies Washington that it will not renew the pact when it expires May 14.

Cajal, the Foreign Ministry political director, told reporters that despite a new U.S. offer on force reductions, difficulties "obviously" remain, and therefore the government will proceed with the notification "in the form and at the time it judges opportune before Nov. 14." If an accord is not reached within six months after that, U.S. forces numbering up to 12,500 would have one year in which to withdraw from Spain.

Both sides emphasized their desire to work out a new treaty. Next week's notification thus ushers in a period of increased pressure in the talks rather than the imminent departure of U.S. military bases, Spanish and U.S. officials said.

"I want to reiterate the common desire on both sides to overcome the difficulties that obviously still exist, to reach a solid bilateral defense relationship for our common interest and to come up with a bilateral agreement satisfactory to both sides within the time we still have," Cajal declared.

"In the recent round we have confirmed once again that both sides want a strong defense relationship and a new defense agreement," Bartholomew said. "With this goal in mind, we are going to continue negotiating and have agreed to hold the next round in mid-December."

Knowledgeable sources said the United States formally proposed yesterday that one-third of the U.S. 401st Tactical Fighter Wing's 72 F16 aircraft at Torrejon Air Base near Madrid be withdrawn from Spain as part of the new agreement. The 24-plane reduction would have to be accompanied by unspecified conditions that guarantee there is no harm to overall North Atlantic Treaty Organization defense capabilities, they added.

Spain has focused its demands on the planes at Torrejon, insisting from the beginning that all 72 U.S. aircraft be removed from the country. Informed earlier of the new U.S. proposal, Defense Minister Narcis Serra dismissed it as "insufficient," and other Spanish officials said the U.S. side still has a "way to go" before agreement is possible.

A U.S. official described the offer as "substantial" and recalled it was the second U.S. concession in the seven rounds of talks since July 1986. The first was in February, when Washington offered to move the F16s to another U.S. facility in Spain as a way to diminish their political visibility by getting them away from Madrid.

Spanish officials said they have stuck to their original demand on the F16s throughout the talks and will continue to do so. In return, they have not questioned the major U.S. naval base at Rota, U.S. air installations at Moron and Zaragoza, and nine other, smaller facilities on Spanish soil, they said.

A high-ranking aide to Prime Minister Felipe Gonzalez said the Spanish leader explained his position to President Reagan before the negotiations began. Nonetheless, he said, U.S. negotiators still expect Spanish concessions on the F16s in return for the proposed U.S. concessions.

The Spanish government takes the position that Spain's NATO membership and continued use of the other U.S. military facilities in Spain are sufficient counterweights to the disruption that would be caused by redeploying the F16s outside Spain, he said.