Russia planned to extend into Monday a marathon salvage operation to raise a sunken riverboat from the muddy bottom of the Volga River a week after it sank killing at least 114 people. An elaborate operation involving dozens of divers and two huge cranes in the central region of Tatarstan began on Saturday and lasted throughout Sunday. The actual lifting of the Bulgaria cruise ship was unlikely to begin before Monday however, contrary to earlier expectations. The 78-metre (255-foot) boat lying on its right side must be put on an even keel before it can be raised, officials said. Those works began at 0300 GMT Sunday and were continuing as of 1500 GMT, emergencies ministry spokesman Andrei Rodygin told AFP. "The works are continuing," he said, refusing to give a precise timeframe for the lifting operation. "A decision has been made to conduct an additional diving exploration," Rodygin said regarding the delays. Divers have been working in a low-visibility environment and the boat was increasingly getting sucked into the riverbed silt, officials said. Bad weather and thunderstorms expected in the region over the next few days are likely to complicate the operation further, they said. The overcrowded 56-year-old boat went down last Sunday in a storm in the country's worst boat disaster in recent memory, which the Kremlin blamed on safety breaches. Raising sunken vessels is considered difficult and expensive. Emergencies ministry officials could not say how much this operation would cost. "No two salvage operations are alike. They are all unique," Rodygin said in televised remarks. "Russia has not seen such a large-scale catastrophe on the water in the past 20 years." The divers were also expected to search the riverbed near the boat in the hope of finding the bodies of 15 people who are still listed as missing. Authorities have so far found the bodies of 114 people including 28 children. Forty-nine divers and two huge floating cranes were involved in the lifting operation, while more than 800 personnel and 23 river vessels have been deployed in the overall search effort. Also Sunday, authorities bestowed medals on the captain and key crew members of the Arabella boat that picked up survivors from the Bulgaria. Transport Minister Igor Levitin has expressed hope that after the Bulgaria was raised officials would obtain additional clues that could help the investigation. Prime Minister Vladimir Putin earlier this week made a surprise visit to the city of Kazan to which the boat was sailing when it sank. Putin laid roses at the river port and blamed greed and safety violations for the accident. The Bulgaria was built in the former Czechoslovakia in 1955 and its sinking prompted Russian President Dmitry Medvedev to call for a river transport safety review. The Bulgaria sank three weeks after a Tupolev-134 plane crashed on a highway in bad weather in the northern region of Karelia, killing 47 people and drawing fresh attention to the dire state of Russia's infrastructure. Putin was president in August 2000 when the Kursk nuclear submarine sank in the Barents Sea claiming the lives of 118 seamen. The wreck of the submarine was raised from the seabed a year later.