Pro-Iranian Muslim extremists holding 13 Western hostages have moved the captives from slums in southern Beirut to a region in eastern Lebanon, security sources said Monday.

The report coincided with a new drive by the United States and Britain to secure the release of the captives through the good offices of Iran and Syria. Iran was officially neutral in the six-week Persian Gulf war, while Syria fought alongside the allies.The sources, who spoke on condition of anonymity, said the captives were now believed held in hideouts controlled by Shiite fundamentalists in and around the ancient Roman town of Baalbek in the Bekaa Valley of east Lebanon.

"The army command is convinced that none of the hostages is in Beirut or its environs any more," one source said. "Lebanese troops searched all suspected hideouts controlled by Shiite fanatics in south Beirut last week and there was no trace of any of the captives."

The source said the search included the two main barracks used by the fundamentalist Hezbollah, or Party of God, in the shantytowns of Hajjaj and Hay Madi, where most of the missing Westerners had been believed imprisoned.

The missing men are six Americans, four Britons, two Germans and an Italian. The Americans are journalist Terry Anderson, educators Thomas Sutherland, Jesse Turner, Alann Steen and Joseph Cicippio, and writer Edward Tracy. Anderson, chief Middle East correspondent for The Associated Press, is the longest held. He was kidnapped March 16, 1985.

The army's search was primarily to ascertain that Christian and Muslim militias that fought Lebanon's 16-year-old civil war have actually pulled heavy weapons from the capital, as pledged under a plan to demilitarize the city. The withdrawal on Dec. 3 was part of President Elias Hrawi's effort to enforce an Arab League-brokered peace plan.

The search operation also established that there apparently have been no violations of the plan within the greater Beirut area, another source said. He said military intelligence believes the hostages were smuggled to the Bekaa before the militias withdrew in December.