South African President Pieter W. Botha's refusal to spell out concrete steps for easing his country's apartheid policies seems certain to increase congressional pressure on President Reagan to sign an economic sanctions bill that is awaiting final Senate approval, congressional sources said yesterday.

Since Congress is in recess until after Labor Day, reaction was relatively sparse in the immediate aftermath of Botha's speech yesterday to a regional meeting of his National Party. But, the sources said, his vagueness and defiant tone clearly disappointed members of Congress who had hoped that he would announce remedial measures to ease South Africa's racial unrest.

House Foreign Affairs Committee Chairman Dante B. Fascell (D-Fla.) summed up what is expected to be the dominant reaction.

He said Botha's approach was "tragically more of the same" and called on President Reagan to abandon his "ineffective policy" of friendliness toward South Africa and align the United States with an international program of sanctions. The House and the Senate have overwhelmingly passed legislation that would impose significant economic sanctions, including curbs on bank loans and new investment, on South Africa, but final Senate action on a conference report is not scheduled until Congress reconvenes.

Senate Foreign Relations Committee Richard G. Lugar (R-Ind.), active in efforts to achieve a bipartisan approach toward a sanctions bill, has urged Reagan not to veto the legislation if South Africa refuses to take initiatives to improve the political, social and economic status of its majority black population. A spokesman for Lugar said yesterday that the senator wants to study Botha's speech before commenting on it.

Sen. Lowell P. Weicker Jr. (R-Conn.), an advocate of a tough line toward Pretoria, called Botha's speech "a vacuous attempt to take the world's eye off the ball. I would hope the Senate's response would be quick passage of the sanctions bill and, if need be, an override of a presidential veto."

Sen. Charles McC. Mathias Jr. (R-Md.) said, "I am not satisfied . . . . I had hoped President Botha would announce some concrete steps to bring equality and justice to South Africa, but I did not find any such steps outlined in his speech."

"President Botha has dashed all real hope that the South African government is ready to change its racist ways," Sen. Edward M. Kennedy (D-Mass.) said. "Let us send a clear and unmistakable message that the time for constructive engagement with racism is over and that the time for firm American action against apartheid has come."

D.C. Del. Walter E. Fauntroy (D), interviewed on Cable News Network, said, "I fear South Africa's government continues its blind march toward tragedy, bloodshed and violence. We now have no choice but to move ahead vigorously" with sanctions.

Sen. Nancy Landon Kassebaum (R-Kan.), chairman of the Senate Foreign Relations subcommittee on Africa, said Botha presented "vitally important issues in a very unclear manner . . . . I fear it will only add more fuel to the fire of those who wish to use violence as a solution. The time bomb of apartheid continues to tick."