The United States today took its first significant step toward easing economic sanctions against Poland by agreeing in principle with other Western creditor nations to resume talks on rescheduling the country's huge foreign debt.

The decision appears to mark an easing by the Reagan administration of its originally tough stance following the imposition of martial law in December 1981 and the crushing of the independent Solidarity trade union. If debt negotiations with Warsaw are successful, they could pave the way for fresh Western credits for Poland's virtually bankrupt economy.

Participants at a private one-day conference of government representatives from 16 Western countries cautioned, however, that there was some way to go before formal talks on rescheduling Poland's official debt are likely to begin. A further "technical" meeting of the group is planned for September, according to French officials. There were conflicting reports on whether Poland would be invited to attend.

State Department spokesman John Hughes said in Washington that the United States "agreed in principle to proceed with the process of rescheduling" but that it remained "deeply disturbed" over the passage of tighter controls over political dissent in Poland, The Associated Press reported.

"The question now is how these laws will be implemented," Hughes said. He added that "the actual release of the vast majority of political prisoners will play a critical role in determining our future policy toward Poland."

Poland owes Western governments and banks $26.6 billion, according to Wharton Econometric Forecasting Associates. About $14 billion of this sum consists of credits guaranteed by governments, including $1.8 billion to the United States, while the remainder is owed to private banks.

The United States had been one of the last countries to oppose a resumption in contacts with Poland over the rescheduling of official debt. Most Western European governments opposed continued suspension of negotiations since they were being required to keep up the high interest payments on the guaranteed credits to private institutions.

Agence France-Presse reported that some neutral countries, including Switzerland and Austria, had hinted that they were prepared to resume direct negotiations with Warsaw whether or not the United States lifted its veto on new talks.

Western officials are likely to point to the Polish government's action in lifting martial law last week and the subsequent amnesty for some political prisoners as a suitable reason for the easing of sanctions. It is clear, however, that sanctions have failed to achieve their original aim of forcing the Polish authorities to end political repression.

In a speech to the Polish Parliament last week, Gen. Wojciech Jaruzelski, the country's military leader, mocked the West's so-called "carrot-and-stick policy" by saying that "the stick proved too short and the carrot not fresh enough."

The sanctions were ordered at the urging of the United States at a meeting of foreign ministers of the North Atlantic Treaty Organization in Brussels on Jan. 11, 1982. A communique issued then listed three conditions for their removal: the ending of martial law, the release of those arrested and an immediate resumption of the government's dialogue with Solidarity and the Roman Catholic Church.

Of these conditions, the first has nominally been met, but many of the repressive measures of martial law have been incorporated into new emergency legislation. The second has been met partially with the release of political prisoners and several hundred people convicted of crimes under martial law. But many leading dissidents and Solidarity activists remain in jail.

As for the third condition, the Polish government has said it is quite prepared to talk to the church, but it rules out any dialogue with the formally disbanded trade union.

During a press conference on June 28, President Reagan promised to relax sanctions if the Polish authorities allowed "a free union that is not subject to government control." The emergence of a trade union similar to Solidarity is, however, now legally impossible in Poland.

Several other U.S. sanctions against Poland remain in effect, including a ban on economic assistance and the sale of high technology. Poland also remains one of several Eastern Bloc countries not to benefit from most-favored-nation trading status with the United States.

Western economic experts say that, of all the sanctions against Poland, the refusal to grant new credits was the only one that had any real bite. Starved for western currency, Poland was forced to drastically cut back its imports of essential components and raw materials and its industrial production fell sharply.

The Polish government has said it will present the Reagan administration with a bill for $6 billion which, it claims, represents its losses due to the sanctions. The figure is regarded as a wild exaggeration by most western economists.

Western private banks continued to negotiate with the Polish authorities over debt rescheduling throughout the period of martial law. The two sides reached an agreement November allowing Poland to postpone repayment of all but 5 percent of the principal on the 1982 debt. Poland agreed to pay back interest worth $1.1 billion, but immediately received half of that back in the form of new credits.

Banks are reported to be close to reaching a similar agreement with Poland this year following talks in Vienna.

In addition to the United States, the countries attending today's meeting were West Germany, France, Britain, Austria, Belgium, Canada, Denmark, Spain, Finland, Italy, Japan, Norway, the Netherlands, Sweden and Switzerland.