Guwahati, Jan. 2 :    Guwahati, Jan. 2:  Seven persons were picked up for interrogation today after suspected Ulfa militants blew up an ONGC pipeline in Upper Assam's Sibsagar district late last night causing fire and oil spill. According to Sibsagar deputy commissioner L.N. Tamuly, oil supply through the line has been restored this evening. The total monetary loss suffered by the ONGC because of the blast has been estimated at Rs 5 lakh. The blast, triggered by a time device, took place on the banks of Disangpani river. It damaged 10 ft of the pipeline carrying crude from Moran to Lakwa. The bomb was strategically placed near the bridge of the river. Sibsagar police officials said the blast took place at 10.30 pm at Saphrai village under Kakotibari police station in Saraihat sub-division of Sibsagar district, damaging the pipeline. No casualty was reported as the area was isolated with no human habitation nearby, Tamuly said. A major fire broke out at the site because of the oil spill and it took fire fighting personnel almost three hours to douse the blaze. 'The sound of the blast could be heard from the Lakwa ONGC colony, which is nearly six km from the site,' officials said. Senior police and ONGC officials have rushed to the spot to take stock of the situation. A combing operation has been launched to nab the culprits, police sources said. ONGC officials said it would take a day to repair the pipeline. Security measures have been intensified in the area. The official said as the line belonged to the central trunk firm (CTF) which had no connection with supply of oil to the refineries, normal activities of the ONGC were not affected. The blast followed a recent announcement by the banned organisation to carry out subversive activities this month, Tamuly said, adding that the information was received by the district administration from a 'secret source' and the authorities were working to ascertain its veracity and initiate counter measures .