The administration's Strategic Defense Initiative Organization is seeking an additional $100 million in research funds to accelerate underground tests of nuclear space-based weapons despite President Reagan's persistent description of the project as a "non-nuclear" defense shield against Soviet missiles, according to U.S. officials.

The request and a pending nuclear test of the controversial X-ray laser has provoked new congressional scrutiny of the project and raised new questions by critics about the true nature of President Reagan's strategic defense program, popularly known as "Star Wars."

The main promoter of the project, which seeks to harness missile-killing laser beams from a nuclear detonation in space, is physicist Edward Teller, often called the father of the hydrogen bomb. According to published reports, Teller convinced Reagan of the need for additional funds to hold more tests.

A government official said that $62 million of the additional $100 million the SDI office is seeking requires explicit congressional authorization to transfer money from the Defense Department budget to that of the Department of Energy because it involves additional nuclear tests of the X-ray laser.

Authorization to transfer the money is required because of legislation prohibiting the Defense Department from carrying out nuclear tests or from moving any of its funds to another department without congressional approval.

However, the SDI office has already shifted $38 million to the Energy Department on "a reimbursement basis" for other nuclear-related research, such as space power plants, for the Star Wars project, according to congressional sources, who indicated that such a temporary shift was permissible.

SDI director Lt. Gen. James A. Abrahamson said last week that he was seeking the additional $100 million for tests to determine whether the X-ray laser was feasible for either the United States or the Soviet Union.

Sen. Ted Stevens (R-Alaska) said in a Senate debate this week that the program is necessary because "we need to have the capability to understand that technology for the simple reason that the Soviets already have a considerable capability in that area."

An SDI official said this week that the extra $100 million, which the office had originally sought from Congress, was in addition to the $282 million Congress has appropriated this fiscal year to the Department of Energy for SDI-related nuclear research. This is in addition to the $2.5 billion to $2.7 billion for research appropriated to the SDI office this year.

Last week, 30 House members wrote a letter to Defense Secretary Caspar W. Weinberger asking that a $30 million test of the X-ray laser being planned by Lawrence Livermore Laboratory in California this month be postponed until "serious flaws" in the design of the experiment are eliminated.

"We are disturbed that at this time of skyrocketing deficits and cutbacks in defense spending money is being wasted in a test that does not provide accurate data," said the letter, which was organized by Rep. Bill Green (R-N.Y.) and Edward J. Markey (D-Mass.).

The congressmen asked for a classified briefing on the X-ray laser program and "a full discussion" of the reported design flaws.

Unlike some critics of the project, the congressmen did not question whether the research program is contrary to Reagan's non-nuclear assertions.

The president, in his Nov. 21 speech to Congress upon returning from the Geneva summit, said he had explained to Soviet leader Mikhail Gorbachev that "we are investigating non-nuclear defensive systems that would only threaten offensive missiles, not people." According to one count, Reagan has referred to Star Wars as "non-nuclear" 17 times in the past year.

The SDI official said that while the antimissile shield as now envisaged was "principally" a non-nuclear system, his office had never concealed that it was also conducting research into the feasibility of using nuclear-generated power in space and the X-ray laser weapon.