The army says it on the verge of completely defeating the Tamil Tiger rebels, who are now cornered in the small stretch of land.
The BBC's Anbarasan Ethirajan says that it is not clear whether these craters are the result of recent fighting or of earlier confrontations between the two sides. Independent journalists are not allowed to enter the war zone.
Our correspondent says that the images will bring more international pressure on the Sri Lankan government to address the plight of the civilians trapped in the safe zone.
One photograph taken by Unosat - the UN's Operational Satellite Applications Programme - and dated 19 April shows what it says is "the suspected site of multiple shelling craters within the CSZ (Civilian Safety Zone)".
The UN has regularly expressed concern for Sri Lanka's civilians
The government has consistently denied targeting civilians and this week said it would not use heavy weapons in the safe zone.
Sri Lankan Foreign Secretary Palitha Kohona said: "This area has witnessed heavy fighting between the two sides in the last 15 years.
"Since then it has been a Sea Tiger (Tamil rebel) base as well. There is nothing to indicate when those shells fell into the region. There is nothing to indicate that the government forces were responsible for these explosions."
Military spokesman Brig Udaya Nanayakkara told the BBC: "We don't fire into the safe zone with heavy weapons. There have been so many explosions inside the safe zone, for example there have been a number of suicide attacks by the Tamil Tigers there.
"We completely deny that the security forces have shelled the safe zone."
Other UN pictures show shelling and bombing damage in and around the village of Vadduvakallu - located immediately south of the safe zone.
Unosat says that they show how the area looked before it was attacked and how it looked afterwards, with "major building destruction".
The majority of the damage occurred between 5 February and 19 April 2009," a UN spokesman said. "We estimate over 150 buildings have been destroyed here."
The UN has repeatedly expressed concern for the tens of thousands of civilians trapped in the zone or in government-run camps outside.
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