PARIS--Secretary of State James F. Byrnes and Russian Foreign Minister Viacheslav Molotov held a surprise one-hour conference today and diplomatic quarters believed that Molotov might have received fresh instructions from Moscow on the future of Trieste, biggest of the many problems which the Big Four foreign ministers face.

The delegates, at a morning meeting, decided that Italy should keep the Pustertal area which Austria has demanded. After lunch Molotov telephoned to Byrnes at the Hotel Meurice, asking for a conference. They talked from 4 P.M. to 5 P.M., when they motored to the Luxembourg palace for a plenary meeting which lasted nearly three hours.

Counting the two Big Four meetings and their own talk; Byrnes and Molotov were engaged in conferences for 6 1/2 hours during the day.