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Unverified footage said to show government troops firing towards Tamil Tiger positions on Monday was aired on Sri Lankan TV station Derana
The United Nations says that 52 civilians have been killed in the past day of fighting in Sri Lanka.
A UN spokesman also said that cluster bombs had hit a hospital, which has been subject to several attacks.
The military denies responsibility for the attacks. There has been no comment from the Tamil Tiger rebels.
Meanwhile President Mahinda Rajapaksa said the rebels would be "completely defeated in a few days" and asked people who had fled to return.
The 52 civilians the UN reported killed were in Sudanthirapuram, in Mullaitivu district. The organisation said another 80 were injured.
Some reports said a makeshift hospital had been hit there but this has not been verified.
Army spokesman Brig Udaya Nanayakkara told the BBC: "Confrontations are taking place in the Sudanthirapuram area. The rebels remove weapons from their dead cadres and put their bodies in civilian areas to show that civilians were killed."
The UN also said the last major hospital in rebel-held territory - in the town of Puthukkudiyiruppu - had been hit by cluster bombs.
Mr Rajapaksa said his government had nearly "destroyed terror"
It said the hospital had been evacuated after 16 hours of shelling. It is not clear who fired those shells.
Sophie Romanens, from the Red Cross, told the BBC more than 300 patients and 20 staff had moved to a community centre in Puttumatalan to the north-east.
The area has a lack of clean drinking water and the situation for the patients was critical, she said.
"They are trapped in a very small area which has come under fighting, and they just have no safe place to go, they cannot escape, and they have lost the means that they need for survival."
The director general of health services Ajith Mendis has told the BBC's Sinhala service that there will be a halt in the conflict on Thursday to allow the wounded to be brought to Vavuniya.
Dr Mendis said the health secretary had told him the defence forces would halt the conflict for a "few hours".
Brig Nanayakkara told the BBC that the military was not responsible for the Sudanthirapuram and Puthukkudiyiruppu incidents.
He said the army did not have "the facility or capability to deliver cluster bombs".
Cluster bombs are controversial as many of the bomblets do not explode and remain a danger to civilians.
The UN says it accepts assertions by the government that it has not used cluster bombs.
Claims by either side cannot be verified as independent journalists are not able to reach the front lines.
Call for return
Separately, in a speech to the nation on the country's national day, President Rajapaksa said the rebels' defeat was imminent.
"Today we have been able to nearly destroy terror," he said.
"At this moment I urge all Sri Lankans from all communities who fled the country because of the war to return to their motherland."
1976: Liberation Tigers of Tamil Eelam form in the north-east
1987: India deploys peace-keepers to Tamil areas but they leave in 1990
1993: President Premadasa killed by Tiger bomb
2001: Attack on airport destroys half Sri Lankan Airlines fleet
2002: Government and rebels agree ceasefire
2005: Mahinda Rajapaksa becomes president
2006: Heavy fighting resumes
2009: Army takes main rebel bases of Kilinochchi and Mullaitivu
The government and rebels have come under more pressure to declare a ceasefire.
This will allow casualties to be evacuated from the war zone in the north-east. Up to 250,000 civilians may be trapped.
A joint statement issued following a meeting between the US Secretary of State, Hillary Clinton and the British Foreign Secretary, David Miliband, said there should be a truce so that humanitarian aid could be allowed in.
The Sri Lankan government has previously ruled out any ceasefire and has vowed to crush the rebels.
The Tigers have said they will not lay down their arms until they have a "guarantee of living with freedom and dignity and sovereignty".
Earlier Sri Lanka's key international donors, the United States, Japan, Norway and the European Union called on the Tamil Tiger rebels to consider laying down their arms to avoid more civilian casualties.
The BBC's Ethirajan Anbarasan in Colombo says this is the first time the influential quartet has issued such an appeal to the Tamil Tigers.
It is also the first international acknowledgement that the rebels may be near to defeat.