At a 2010 U.N. climate convention governments had agreed to take "urgent action" to meet the target but chief U.S. climate negotiator Todd Stern recently said insisting on the target would lead to "deadlock."
Now the European Union and the Alliance of Small Island States have responded by saying the Unites States should stick to promises made.
"Suddenly abandoning our agreement to keep global warming below 2C is to give up the fight against climate change before it even begins," said Tony de Brum, minister in Assistance for the Marshall Islands.
"'Flexibility' on our 2C limit would set the world on a path to irreversible, runaway climate change.
"For many low-lying island states, including my own, that is not a solution -- it is a death sentence," de Brum told BBC News.
EU climate spokesman Isaac Valero-Ladron said governments -- including the United States -- had a responsibility to live up to prior agreements.
"Also, consolidated science continues to remind us of the dire consequences of going beyond such a temperature increase," he said.