The United States has come under pressure to follow other rich countries and set a 2020 goal for cutting greenhouse gases to rescue chances for a deal due next month at the Copenhagen summit.
The prospective Danish hosts continued to put pressure on the US at a final preparatory meeting in Barcelona, saying it could not come "empty-handed" to Copenhagen, reports Reuters.
Meanwhile, some African countries threatened to walk out of the Barcelona talks yesterday, saying rich countries had to deepen their emissions-cutting targets.
The head of the UN Climate Change Secretariat said a figure from the US was essential, reports Reuters.
"We need a clear target from the United States in Copenhagen," Yvo de Boer told a news conference.
President Barack Obama, speaking at the White House yesterday, held out hope for "an important deal" in Copenhagen.
But he said such a deal might not solve "every problem on this issue, but would nevertheless take an important step forward.
The United States has yet to offer a firm target for reducing emissions by 2020.
In contrast, the European Union has promised a cut of at least 20% below 1990 levels by 2020.