US-based Human Rights Watch says the United Nations should send observers to monitor the fighting in Sri Lanka.
The mission of Nordic ceasefire monitors there ends on 16 January following the government's decision to withdraw from a ceasefire.
On Wednesday the government said the 2002 ceasefire deal was moribund.
Thousands of people have been killed in the last two years with the military and the Tamil Tigers accused of widespread human rights abuses.
The Nordic-led Sri Lankan Monitoring Mission (SLMM) was set up as part of the 2002 agreement to investigate violations of the accord.
It says that, because of the current level of fighting, it is only able to look into major incidents.
A spokeswoman was quoted by Reuters news agency as saying: "We're beginning to move towards a pull-out... It's not far away, it's only a few days."
'Greater than ever'
In a statement issued on Friday, Elaine Pearson of Human Rights Watch said, "The Sri Lanka Monitoring Mission was deeply flawed, but its monitors helped to minimise abuses against civilians.
"Now the need for a UN monitoring mission is greater then ever."
The US State Department has joined in the international concern at the end of the ceasefire.
The move would "make it more difficult to achieve a lasting, peaceful solution to Sri Lanka's conflict", spokesman Sean McCormack said.
"We call on both the government and the Liberation Tigers of Tamil Eelam (Tamil Tigers) to avoid an escalation of hostilities and further civilian casualties."
About 5,000 people have been killed since early 2006, taking the total number of dead since the war erupted in 1983 to around 70,000.
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