U.S. intelligence organizations have concluded that President Ali Abdullah Saleh's government in North Yemen, which is receiving emergency shipments from a $500 million U. S. arms package, is likely to collapse within the next six months, according to U. S. sources.

The intelligence estimates circulating in the administration concede that U.S. analysts are highly uncertain who will succeed the erratic, unpopular Saleh and take control of the F5E jet fighters, M60 tanks and other weapons now being turned over to the North Yemen army, according to these sources.

The new estimates of Saleh's shaky grip on the country he has ruled since June, and the uncertainty about his successor, have touched off discussions at the State Department, National Security Council and Pentagon about slowing the emergency shipments authorized by President Carter on March 9 to stem an invasion from Marxist-ruled South Yemen.

But Pentagon officials said there has been no change in the schedule of deliveries authorized under Carter's decision to use a presidential waiver to bypass congressional review of a large arms export. The border war between the two Yemens began to die down aboutt the time of Carter's decision and has been quiet since.

The Carter administration has been careful not to identify its military and diplomatic support for North Yemen specifically with Saleh, whose two predecessors were assassinated within 18 month of one another and who has faced two coup attempts.

But the administration's actions on the Yemeni crisis have indicated a large American interest in stability in that mountainous Arabian Peninsula country, and Saleh's overthrow could have implications far beyond its border.

The administration's actions in the Yemeni conflict were intended more to reassure conservative Arab governments-especially Saudi Arabia-that Washington Would act to protect them and to signal the Soviet Union that "the limits of destabilization" had been reached in the Persian Gulf, according to a senior U.S. official.

Administration officials assert that those objectives were achieved by rushing the Saudi-financed arms package to North Yemen, deploying the aircraft carrier Constellation into the Arabian Sea, sending two Boeing radar-equipped airborne warning and control systems (AWACS) aircraft to Saudi Arabia, and taking other steps.

Administration sources said there was concern that an unchallenged and visibly successful invasion by South Yemen would have a devastating impact not only on North Yemen but also on Saudi Arabia, Oman and other friendly countries. Senior administration planners contemplated using the Constellation and its warplanes to halt the flow of Soviet arms to South Yemeni forces, if necessary, to contain the invasion. The sources would not specify if operational plans for using U.S. military force were drawn up during the crisis.

Senior U.S. policymakers reportedly feel, however, that quick Soviet acquiesence in an Arab League-sponsored cease-fire was more influenced by Soviet desires not to antagonize Saudi Arabia than by the implied U.S. military threat.Riyadh and Moscow have sent out feelers in recent months about improving diplomatic relations.

U.S. analysts conclude that the South Yemeni military thrust into North Yemen was not an attempt to occupy the country, but was aimed at destroying Saleh's credibility and embarrassing his army so severely that it would turn against him. U.S. intelligence reports cite growing opposition to Saleh within the army and among the nation's tribal leaders since the February fighting.

Under the arms package covered by Carter's waiver, a squadron of 12 F-5Es has been delivered to Saudi Arabia for reassembly and shipment to North Yemen. Saleh's government also has received artillery, armored vehicles and 32 M60 tanks through Saudi Arabia and is receiving additional U.S. equipment from Jordan.

A second group of 32 tanks consigned to North Yemen is en route by sea, and undetermined amounts of artillery, Vulcan antiaircraft guns, TOW and Dragon antitank missiles and other weapons are still to be shipped.