Insurgent Afghan tribesmen, tired of the bickering and lack of unity among the major rebel groups here, today took the first big step toward forming a government-in-exile that they said would represent the entire Soviet-occupied country.

There were indications tonight that at least two of six major Peshawar-based rebel groups would join in the new movement, which plans to seek the support of the Islamic foreign ministers' meeting that opens Wednesday in Islamabad. o

The new movement's major claim to legitimacy is the way it was born -- out of the grand tribal council that traditionally is the ultimate authority in Afghan political life. There have been only six such councils held in this century, and this is the first one ever to be held outside of Afghanistan.

According to council leaders, 916 tribesmen came from every district in Afghanistan, traveling by foot, bus and camel through areas where rebels and Soviets are engaged in heavy fighting. Each of Afghanistan's 216 districts had three representatives who had to provide documents from their neighbors naming them as delegates. Other representatives came from nomadic tribes in Afghanistan.

In all, the council included 38 former members of the Afghan parliament and Mohammed Babrak Zai, a former chief justice of the Afghan Supreme Court, was elected council president.

Nonetheless, this movement is likely to further confuse the already splintered Afghan rebel scene here. The inability of the rival rebel factions to unite has kept Western and Islamic nations from giving military and economic help to them to fight the Soviet-backed government of Babrak Karmal.

Last January the Islamic foreign ministers forced the rebel groups to join in a National Alliance for the Liberation of Afghanistan, but that group soon fell apart.

All six Peshawar-based rebel groups have been given until May 21 -- coincidentally the final day of the Islamic foreign ministers' conference -- to renounce their independence and join the new movement.

If they do join, they would each be allowed seven seats on the revolutionary council, which would act as the executive branch of the government-in-exile.

Each of Afghanistan's 28 provinces would have two representatives on the revolutionary council who would stay here to run the government-in-exile. But the rest of the delegates would return to their home districts to form underground provincial councils to coordinate the antigovernment activities -- something that has been sorely lacking among the rebel groups.