Heavy exchanges of gunfire briefly interrupted a two-day-old truce late Thursday between Lebanese troops besieging a Palestinian refugee camp and Islamic militants holed up inside. The renewed fighting on the northern edge of the Nahr el-Bared camp appeared so far limited to exchanges, which lasted 20 minutes before tapering off into sporadic gunfire, and there was no sign that the troops positioned on the camp's edge were making any move to enter. Until sundown Thursday, only sporadic gunfire marred the truce. Security officials said army positions came under heavy machine gun fire from Fatah Islam gunmen followed by rocket-propelled grenades, so the army "dealt" with the source of fire. The officials, speaking on condition of anonymity because they were not authorized to speak to the press, said the army will not initiate fire but will respond to attack.