Six Lebanese Army soldiers were killed early today and 12 others wounded in a clash that apparently involved pro-Iranian Shiite Moslem militiamen, who had warned the Army against conducting exercises in eastern Lebanon's Bekaa Valley.

The deaths came in an ambush and three-hour firefight that occurred as the Army force was heading to a rifle range near the Roman ruins in the ancient city of Baalbek, according to state and private radio here.

An Army spokesman privately told reporters here that the attackers were pro-Iranian militiamen and Iranian Revolutionary Guards. Army spokesmen said three of the band of irregulars were killed and five wounded in the clash, which was halted by the Syrian Army.

Syrian forces control the eastern Bekaa Valley. As is the case in Israeli-occupied southern Lebanon, Lebanese Army soldiers maintain barracks there but make no effort to exert authority.

Also in the area are the pro-Iranian Lebanese Shiite Moslems and a contingent of about 300 Revolutionary Guards from Iran's Shiite-dominated Islamic government.

The Shiites have proclaimed the area to be a separate Islamic state. Last November, hundreds of armed Shiite demonstrators stormed the city hall of Baalbek and virtually took over the town.

Recently, the Shiite Moslem militia issued a warning to the Army against holding any exercises near Baalbek, according to radio reports. No reason was given for the warning.

The Lebanese Army statement said one lieutenant, two corporals and three privates were killed in the ambush. A colonel was among the 12 wounded.