Oxfam has urged the US, Russia, India and Brazil to support a UN reform that would require the organisation to act quickly to prevent genocide.
Oxfam says reform would help stop a repeat of genocide like Rwanda's
The international charity accuses the four countries of blocking UN plans designed to stop atrocities such as the 1994 Rwanda genocide happening again.
Oxfam says the proposal would oblige the international community to take action if governments failed to do so.
Its statement comes ahead of a UN summit next month to discuss reforms.
Oxfam says that while US officials publicly back the planned reform, in principle they are seeking to water it down.
Other countries opposing the move include Syria, Iran, Cuba, Pakistan, Egypt and Algeria, the charity said.
The current draft statement says the UN has a "shared responsibility to take collective action, in a timely and decisive manner" to "help protect populations from genocide, war crimes, ethnic cleansing and crimes against humanity".
Oxfam says it is essential the agreement remains this strongly worded, if UN members are to prevent future genocides happening.
Such a reform would establish a new standard, the charity says, and oblige the international community to act when required.
Oxfam's spokeswoman in New York, Nicola Reindorp, said: "We've taken the step of exposing the governments blocking the agreement so people around the world can call on them to change their minds.
"We urge these governments to urgently reconsider their position and agree to protect civilians from mass murder and atrocities.
"The international community must never again allow genocide or mass murder to go unchecked."
UN Secretary General Kofi Annan has put forward wide reforms
Governments supporting the call for strong language in the draft statement include Rwanda, South Africa, Tanzania, Nigeria, Kenya, Chile, Peru, Argentina, Australia, New Zealand, Japan, South Korea, Singapore, Canada and the EU.
However, opposition from the would-be blockers could still dilute the commitment and so make it meaningless, Oxfam warned.
"Those supporting the responsibility of states to protect civilians must stick to their principles and those opposing it must think again," Ms Reindorp said.
"Brazil, India, Russia and the US must play their part in helping to stop the slaughter of thousands of innocent civilians."