Soviet President Leonid Brezhnev firmly endorsed Poland's new leaders at a Kremlin summit today, saying he is convinced they can resolve their political and economic crisis with continued strong Soviet backing.

Following their five-hour meeting, the official news agency Tass said that the two sides reaffirmed their resolve to strengthen "the indissoluble" ties between the Soviet Union and Poland.

Polish party chief Stanislaw Kania and Premier Jozef Pinkowski, after a mission to Moscow lasting only slightly longer than the meeting and marked by "cordiality and identify of views," according to Tass, immediately flew back to Warsaw to face crucial negotiations Friday with independent union leaders about Communist Party insistence on supremacy in the new labor movement.

Tass said Brezhnev "expressed the conviction of Soviet Communists, that the Communists, working people of fraternal Poland will be able to resolve the acute problem of political and economic development facing them." The Tass account is viewed as Kremlin endorsement for now of Kania's efforts to contain the impact of the independent unions organized after last summer's strikes toppled his predecessor, Edward Gierek. It was Kania's first visit to Moscow since he took over in Poland two months ago.

Informed Western sources here believe the Soviets bluntly told the Poles to find effective ways of controlling the new unions if Kania and Pinkowski are to retain Kremlin confidence. Tass said only that "the Soviet and Polish leaders expressed determination to protect and strengthen the indissoluble fraternal ties linking the two peoples."

Pinkowski on Friday is scheduled to begin negotiations with leaders of the 7-million-strong Solidarity independent union over their threat to call a strike if the authorities do not remove a party supremacy clause from the union's new charter.

The crisis was heightened by a mysterious explosion early this morning in the Warsaw offices of the Soviet airline Aeroflot as well as by stern new East German and Czechoslovak measures to restrict contacts with Poland as a way of walling off the impact of the union movement.

The Soviet media has made no reference to these events. Tass said Kania told Brezhnev and other Kremlin leaders of the "efforts being undertaken by the Polish Communists to stabilize the situation in the country."

While Poland had not commented on the actions of its neighbors, Polish officials have expressed dismay at new travel restrictions that went into effect today. The East German government said these will remain in force until the situation in Poland is "stabilized." s

The East German and Czechoslavak press has also issued strong warnings of a counterrevolution in Poland, but these accounts have been ignored by the Soviet press which has avoided suggestions of this kind. An authoritative East German commentary warned that Poland's allies have a right to expect the country to be "internally secure", presumably suggesting they also have the right to take action to restore orthodox Communist rule there.

Brezhnev and his Polish guests "resolutely condemned" alleged attempts "by certain imperialist circles to wage subversive activities against socialist Poland and interfere in its affairs," Tass said. Soviet jamming of Western radio stations continues, and foreign diplomats in recent weeks have found it virtually impossible to obtain Soviet permission to cross into Poland at Brest, a major rail link between the two countries.

Most foreign sources view the hurried parley as a major Kremlin move to pressure the Poles, review their strategy for dealing with the Polish union movement and probably give Kania, a tough former internal security chief, reason to tell the union leaders why the party must retain ultimate control of their movement. Last month, the Soviet Communist Party paper Pravda quoted Lenin as saying no union could be allowed to work outside the framework of party control.

Today's meeting underscores anew the deep anxieties set off within the Kremlin by the Polish crisis, which erupted last summer with worker protests over price increases and now takes its place beside the Moslem rebellion against Marxist rule in Afghanistan as the most serious "fraternal socialist" crisis facing Moscow.

Tass said the two sides today gave "special attention" to undisclosed ways to increase Polish production of industrial and consumer goods. The countries reached an understanding, Tass said, "to work out economic programs on the most important problems for the near future and in perspective." There apparently will be new cooperation on making better farm equipment.

Agriculture is the Soviets' domestic Achilles' heel. After two successive harvest disasters and widening shortages of fresh meat and dairy products here, the Russians have earmarked about 500,000 tons of extra food and consumer goods for the Poles to make up for shortfalls caused by the strikes, and granted a $150 million emergency hard currency credit to Warsaw.