Simon Stiell, executive secretary of the UN Framework Convention on Climate Change, criticized the host nation's security personnel for "failing to act" on Tuesday night when dozens of Indigenous protesters forced their way inside, according to a letter reported by Bloomberg News.

His letter, addressed to Brazilian President Luiz Inacio Lula da Silva's chief of staff and to Andre Correa do Lago, the diplomat presiding over the summit, accused the hosts of a "serious breach of the established security framework.

"It flagged vulnerabilities -- from unsecured doors to the lack of assurances that authorities would respond to intrusions -- as well as health and safety concerns from malfunctioning air conditioning and rainwater from heavy Amazon downpours seeping into lighting fixtures.

In its response, seen by AFP, Brazil said: "Internal security within the Blue Zone is the responsibility of the UN Department of Safety and Security (UNDSS), which defines how all areas within it will be protected."

The response added federal and Para state authorities had worked together with UNDSS to carry out a reassessment, ultimately deciding to increase the number of security personnel, expand a buffer zone, and physically strengthen the perimeter with more barriers.

"It is a non-issue now. The issues related to that have been secured," Correa later told reporters at an evening press briefing.

The Brazilian response also addressed the infrastructure issues.

"There was no flooding of the event venue, only localized occurrences such as leaks" that were caused by broken gutters, it said, adding these had already been repaired.

Additional air conditioning units have also been installed, it said.

The choice of Belem, a mid-sized city on the edge of the Amazon, had been controversial because of insufficient accommodation and fears the city was unprepared to handle such a large event involving tens of thousands of people.

Lula has however defended the choice, saying it was important to bring the world to the Amazon to showcase its struggle and crucial role in combating climate change.