COLOMBO (AFP) - Sri Lankan troops Wednesday seized a key Tamil Tiger base in the island's north while two days of fighting across the region has left at least 77 dead on both sides, the Defence Ministry said. Security forces, aided by Mi-24 helicopter gunships, advanced towards rebel defence lines in the Mannar district, killing 53 guerrillas and injuring 38 on Wednesday, the Ministry said, adding that six soldiers were wounded. The clashes in Mannar saw security forces capture the rebels' "18 base", a statement said. It also said air force fighter jets bombed two rebel artillery gun positions in the northern region of Weli Oya on Wednesday. Another 23 rebels were shot dead during ground battles across the Jaffna, Vavuniya and Mannar regions on Tuesday, the statement said, adding that one soldier was killed and four hurt. There was no immediate comment on the ministry casualty reports from the Liberation Tigers of Tamil Eelam (LTTE), who have been fighting government troops since 1972 for a separate state for minority Tamils. However, the LTTE accused the military of shelling a civilian settlement in the north on Wednesday, killing a 52-year-old man and critically injuring a 32-year-old woman. "Two hours of incessant shelling was directed at the civilian settlements and it is lucky that more people were not killed or injured," the LTTE peace secretariat said. The Tigers also said unidentified gunmen shot dead a Tamil medical student in Jaffna on Tuesday night. The latest figures raised the number of rebels killed since January to 3,270, according to the defence ministry, which says 234 soldiers have died in the same period. Casualty figures from both sides vary widely and cannot be independently verified as Colombo bars reporters and rights groups from travelling to the embattled areas. Tens of thousands have died in the conflict, one of Asia's longest running wars. Meanwhile, Sri Lankan security forces arrested a local staff member attached to a United Nation organisation for carrying a pistol with him, the Defence Ministry said. Thiagaraja Prabhakaran, working with the World Food Programme office in the northern district of Kilinochchi, was held while he was being searched at a roadblock in northern Vavuniya, a statement said. The suspect had confessed to carrying the small weapon for someone else in the eastern district of Batticaloa, in return for cash, it added. The UN said in a statement that the aid worker was travelling in a private vehicle outside his working hours at the time of his arrest. "The matter is currently under police investigation while the UN is fully co-operating with the authorities," the UN said in a release.