The Reagan administration has quietly suspended military and economic aid to Panama, rejected a Panamanian government request to buy tear gas for use against public demonstrations and downgraded its contacts with military strongman Gen. Manuel Antonio Noriega as U.S.-Panamanian relations have soured in recent weeks, according to administration officials.

The U.S. actions, none of which have been publicly announced, reflect a growing belief at high levels of the administration that Noriega's domination of Panama is both unacceptable and under greater pressure than at any time since he took power as commander of the Panamanian Defense Forces in August 1983.

Informed sources said U.S. contingency planning has begun for facilitating the departure of Noriega, including the question of where he might go into exile.

A senior official, while saying such matters are being talked about within administration circles,

said no contacts have been made

with other countries that might accept the Panamanian military leader.

There is no consensus within the government, however, that Noriega is actually nearing the end as the most important power in the Central American country, which is vital to the United States because of the Panama Canal.

A U.S. policy-maker said the reaction of Panamanians to Noriega with political protests and large-scale public demonstrations since early June suggests that "Noriega's days are numbered -- but that number could be dozens, hundreds or even thousands of days. If he is as wily and ruthless as everyone says, it will be a difficult situation."

All new commitments of U.S. military and economic aid were quietly suspended early this month after supporters of Noriega, believed to have been orchestrated by the Panamanian government, attacked the U.S. Embassy and consulate in Panama City with rocks and red paint. The cutoff extended to a ban on routine maintenance and repair provided to the Panamanian military by the U.S. Army, the only part of the U.S. action that has been publicized.

As the situation in Panama worsened in recent weeks, and especially after several U.S. servicemen reported prison abuses suffered by anti-Noriega protesters, the aid freeze was tightened to include a ban on U.S. training of Panamanian military personnel and supply of normal spare parts and equipment, officials said.

Initially it had been expected that the aid suspension would be lifted as soon as Panama pays a $106,000 bill presented by the United States for the damage to the embassy. That is expected this week. But in the light of developments, the administration is considering whether to continue part of the aid freeze even after the Panamanian payment is received. Panama is scheduled to receive $6 million in U.S. military aid and $20 million in economic aid this year.

The Panamanian request to buy U.S. tear gas was made last month, before the attack on the embassy, and was turned down by the State Department, officials said.

High-level U.S. visitors such as Deputy Assistant Secretary of State William G. Walker, who made a fact-finding trip to the country last week, have avoided contact with Noriega. "He {Noriega} is seeing fewer American officials," a Washington policy-maker said, adding that close contacts with Noriega at this point "could be misconstrued."

The Reagan administration has long been unhappy about Noriega, who has been dogged for years by charges of involvement in murder, drug activities, electoral fraud and intelligence dealings with Cuban President Fidel Castro. To most officials, however, there seemed little that could be done because of Noriega's firm grip on the Panamanian military and behind-the-scenes control of many civilian activities.

The protests that erupted in early June after accusations against Noriega by a just-retired rival, Col. Roberto Diaz Herrera, were much more widespread and long-lasting than Washington had foreseen, and for the first time involved a broad coalition of opposition groups, including representatives of the business community.

A key recent event was a U.S. Senate resolution June 26 calling for Noriega's suspension pending investigation of the charges against him. The attack on the embassy, which followed shortly after, was said by State Department officials to be part of an effort by Noriega to blame the United States for the recent unrest.

As the turbulence grew, Panama leaped much higher on the agenda of U.S. policy-makers, as Washington pondered how to use its influence to encourage Noriega's ouster in a way that would minimize instability now and after he is gone. At least four meetings of a "restricted intergency group" and one meeting just below Cabinet level have been held on Panama in recent weeks, and Cabinet officials such as Secretary of State George P. Shultz are receiving regular Panama briefings.