DALLAS (AP) _ Federal regulators and the manufacturer of DC-10 jets have rejected a call by an airline passengers group to ground the aircraft to check for structural problems following last week's deadly plane crash in Iowa.

The International Airline Passengers Association said Thursday it wants the Federal Aviation Administration to order the planes out of use in the wake of the July 19 crash of a United Airlines DC-10 in Sioux City, Iowa, that killed 111 people, said spokesman Daniel Smith.

Citing the crash, Smith said it is evident that the aircraft's hydraulic systems, necessary for flight control, do not work and the systems need to be separate.

In a letter to FAA chief James Busey in Washington, IAPA said it wants to ''first, find out what's wrong with the aircraft; and secondly, get it fixed.''

Although IAPA has not ruled out court action, the group, which claims 110,00 members around the world, said it hopes the plane will be grounded without having to resort to legal means.

Jay Leyden, a FAA spokesman in Washington, rejected the demand.

''There's nothing that's come out of the Sioux City accident indicating a basic design flaw that would warrant such an action,'' Leyden said.

Also on Thursday, eight days after the Iowa DC-10 accident, a Korean Air DC-10 crashed short of a fogbound airport in Tripoli, Libya, killing at least 75 people. But a spokesman for the South Korean embassy there said no mechanical problem was reported before the crash.

McDonnell Douglas, maker of the DC-10, held a news conference at the Long Beach headquarters of Douglas Aircraft to denounce the IAPA's call to ground the aircraft.

''I find it difficult to believe that a rational public, a rational FAA and even a rational Congress will put very much credence in the statements of the IAPA,'' said Dale Warren, a McDonnell Douglas vice president.

''The real concern, in my opinion, is that we worry about the credibility of our aircraft to the traveling public,'' Warren said.

Nevertheless, Warren said, engineers are studying the crashes to determine whether a design modification is needed.

''It would appear at this time that some action is appropriate,'' Warren said.

Changes under consideration are additional backup controls, ways of containing the damage from a jet engine explosion and methods of shielding the hydraulic system from such damage.

An initial investigation of the Iowa crash indicates that an explosion in the plane's tail-mounted engine threw pieces of the turbine through the tail section, rupturing at least two of the three hydraulic lines.

McDonnell Douglas technical experts have been sent to Tripoli in hopes of assisting in the investigation of the Korean Air crash there, Warren said.

He said there are no indications to link the two recent crashes other than the make of the aircraft.

After a DC-10 crash in Chicago in 1979, the IAPA won a federal court order grounding the DC-10 fleet for more than a month while inspections were performed on some parts of the aircraft.

IAPA then had asked the FAA to separate the hydraulic systems of the aircraft.

In all, Smith said at least 17, or 3.8 percent, of the 445 DC-10s built have been wrecked because of their problems, a higher percentage of flaws than Boeing 747s or Lockheed L-1011s.