The families of the two top officials of Afghanistan's increasingly besieged regime have been secretly flown to the Soviet Union, apparently for safekeeping, according to U.S. intelligence reports.

Administration intelligence analysts see the sudden departure from Kabul of the entire families of President Nur Mohammed Taraki and his top aide, Hafizullah Amin, in late May as the clearest sign yet that Taraki's rule may be crumbling before a national rebellion against its policies.

The intelligence reports portray a significant increase in the level of fighting in the Afghan countryside in recent weeks, and state that at least three Soviet military advisers accompanying Afghan units have been killed in clashes.

The reports are being studied intently by Soviet specialists at the State Department who are trying to judge the Kremlin's likely reaction to the escalation of fighting inside one of the Soviet Union's most important client states on its border.

The view of these specialists is that an aging and conservative Soviet leadership is not likely to become deeply involved in providing direct military protection for Taraki's year-old government, which replaced that of President Mohammed Daoud after a military coup.

The Soviets now have about 2,000 military advisers in Afghanistan but the extent of their direct involvement in the fighting is not clear to U.S. analysts.

The insurgency is spearheaded by a group of 10,000 to 20,000 guerrillas operating into Afghanistan out of refugee camps in Pakistan. The government is reported to have lost control over at least two central provinces, but no large towns have yet fallen to the insurgents.

The insurgency has spread in recent weeks into previously unaffected towns and villages that are located along Afghanistan's northern borders and which have no direct contact with the guerrillas operating in the southern and central regions, U.S. officials said.

Air strikes have enabled government forces to hold major towns, but the often indiscriminate bombing and strafing are reportedly adding to the popular resentment against the Kabul authorities. The Marxist government's land reform programs and other attacks on the tribal chieftancy system of the countryside have stirred much of the opposition that feeds the insurgency.

The Pakistan-based exile groups are split into at least four major factions that range from monarchists to ultraleftists, U.S. officials said. Analysts here say the rebels have no clear common political objectives and that Taraki's downfall would probably be followed by a lengthy period of instability and maneuvering for power.