By Simon Gardner

COLOMBO, March 6 (Reuters) - Sri Lanka's government is responsible for widespread abductions and disappearances as it fights a new phase in a 25-year civil war with the Tamil Tigers, Human Rights Watch said on Thursday, calling for a U.N. monitoring mission.

Rights groups have reported hundreds of abductions, disappearances and killings blamed on one side or the other since the civil war, which has killed nearly 70,000 people since 1983, resumed in 2006 as a truce collapsed on the ground.

"President Mahinda Rajapaksa, once a rights advocate, has now led his government to become one of the world's worst perpetrators of forced disappearances," Human Rights Watch Deputy Asia Director Elaine Pearson said in a statement, launching a 241-page report.

The report, entitled "Recurring nightmare: State responsibility for disappearances and abductions in Sri Lanka", features interviews with relatives of 'disappeared', some of whom describe how police had paid visits shortly before abductions.

Others describe relatives disappearing after being interrogated at gunpoint by police in broad daylight, or being bundled into white vans by unidentified gunmen. Police then deny their relative has been arrested.

"We absolutely deny the exaggerated allegations reflected in the Human Rights Watch Report," said Foreign Secretary Palitha Kohona. "The report talks about a situation which is getting worse by the minute, and I think it is not only unfair, it is malicious.

"There are all these allegations and they are not exactly serious allegations. They are concocted in our view or exaggerated in order to give the country a bad name," he added. "Over the last 12 months, the situation has improved considerably..., a result of measures put in place by the government."

After formally scrapping a 6-year truce with the Tigers in a wider bid to win the war militarily, Rajapaksa's government banished Nordic truce monitors who had blamed troops and rebels for repeated abuses.

The former Sri Lanka Monitoring Mission enraged the government by accusing state security forces of massacring 17 local staff of international aid group Action Contre la Faim in 2006 -- the worst attack on aid workers since the 2003 bomb attack on the U.N. office in Baghdad.

The government has rejected calls for a U.N. human rights monitoring mission, and has publicly vilified U.N. officials who have questioned its rights record.

Rajapaksa himself has dismissed reports of abductions and disappearances, some from areas under tight military control, as propaganda aimed at tarnishing his government's reputation.

Some government officials have said reported disappearances and abductions were cases of love-struck youths eloping or going on holiday.

Hundreds of relatives of Sri Lankan civilians who have disappeared as the war has escalated demand answers.

"So long as soldiers and police can commit disappearances with impunity, this horrific crime will continue," Human Rights Watch's Pearson said. "The Sri Lankan government's rejection of a U.N. monitoring mission reflects badly on its commitment to human rights.

"While the government dawdles, many Sri Lankans will continue to pay the price."

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