"Peaceful protest is part of any democracy, but blocking access to the United Nations ... as well as intimidating and harassing UN personnel is a breach of international norms and harmful to Sri Lanka's reputation in the world," it said.
The statement was signed by the heads of mission in Sri Lanka for the US, Britain, Germany, France, Italy, Switzerland, the Netherlands, Romania, Norway and the European Union.
"We call upon the government of Sri Lanka to take all appropriate steps to ensure the safety and security of UN personnel and premises," the statement said.
At the same time a statement from Ban Ki-moon, the UN secretary general, called on Sri Lanka to "normalise conditions" around the UN office in Colombo "so as to ensure the continuation of the vital work of the organisation to assist the people of Sri Lanka".
Ban's office said the "strong reaction" to the UN inquiry was "not warranted," adding that the panel of experts had only an advisory capacity.
The panel, the statement said, "has been set up to advise the secretary general... on the modalities, applicable international standards and comparative experience relevant to an accountability process."
Weerawansa, the minister leading the protests, has said the action outside the UN offices is a matter of national pride.
On Friday, he announced he was beginning a hunger strike which he said would continue "until death", or until the UN suspended its investigation.
"His condition is deteriorating, but he is determined," Wasantha Bandara, a spokesman, told reporters.
|Weerawansa has said the protests are a matter of national pride [AFP]
Weerawansa also said he was quitting his post as housing minister, although officials said that Mahinda Rajapaksa, the Sri Lankan president, had refused to accept his resignation.
The UN has said that at least 7,000 Tamil civilians were killed in the final stages of the island's decades-long war last year.
Human-rights groups have accused the Sri Lankan government of committing regular human rights abuses throughout the conflict.
Amnesty International and other groups said government forces intentionally shelled civilians, bombed hospitals and food distribution points, and opened fire in declared no-fire zones.
They also accused the government of shelling hospitals filled with wounded civilians.
The Sri Lankan government has repeatedly rejected the allegations, and it has ignored calls for an investigation.