President Barack Obama will seek to galvanize international support in the fight against climate change on Tuesday when he addresses the United Nations, with time running out on his hopes of leaving a lasting environmental legacy.

Obama has warned that failure to act on climate change would be a "betrayal" of future generations, but faced with a Congress reluctant to even limit greenhouse gas emissions -- let alone ratify an international agreement -- his options appear limited.

Tuesday's climate summit in New York kicks off a process that will culminate in Paris at the end of 2015, where the world's powers will hope to seal a new global climate change pact.

"Internationally, this is the opportunity for the president to leave his mark on the issue," said Alden Meyer, of the Union of Concerned Scientists, a Washington-based think-tank.

Obama's last meeting with heads of state to try to strike a climate deal, in Copenhagen five years ago, ended in bitter disappointment.

"I think that people are justified in being disappointed about the outcome in Copenhagen," Obama said at the time, lamenting the failure to agree a timetable to reduce emissions over the coming decades.

- Complicated landscape -

Five years after that setback, the landscape remains complicated.

"I think that there is some greater sense of perhaps realism as well as ambition among parties than perhaps there was in 2009," said Todd Stern, the United States top climate negotiator.

"I think, at the same time, these negotiations are always difficult," he said in a recent conference call.

In the short term, it remains highly unlikely that the 120 heads of state and government due to attend Tuesday's one-day meeting in New York will meet the expectations of UN Secretary General Ban Ki-moon, who has called for "bold pledges" to be made.

The White House has said it will not announce its post-2020 goals in New York this week, but rather plans to roll an out ambitious target early next year according to John Podesta, Obama's adviser on climate and energy.

"You can expect the US to make public by the first quarter of 2015 a strong national target for reducing greenhouse gas emissions in the post-2020 time frame," Podesta said.

"The President will use his speech at the Climate Summit to call on other leaders to keep their ambition high and to work toward a strong global framework to cut emissions."

For the time being, the Obama administration will highlight the measures it has taken in recent months to reduce the consumption of fossil fuels.

And activists may be gearing up to push harder: celebrities, political leaders and the masses rallied in New York and across the globe on Sunday demanding urgent action on climate change, with organizers saying 600,000 people hit the streets.

- Ambitious 2030 goal -

In June, Obama unveiled new standards aimed at achieving a drastic reduction in carbon emissions from all existing power plants -- a 30 percent reduction of 2005 levels by 2030.

But the White House has delayed addressing the difficult debate surrounding the legal nature of the agreement that 195 nations in the UN Convention on Climate Change will hope to reach in Paris at the end of next year.

The US Constitution states that all legally binding treaties must be ratified by two thirds of the US Senate, an unthinkable prospect in the current political climate. Memories of the Kyoto Protocol, negotiated and signed in 1997 but never ratified by the United States, also loom large.

US negotiator Stern said the terms of any new climate agreement "is a matter that is completely open for question and for discussion," noting that in Durban in 2011 the countries had agreed only to negotiate a "protocol, another legal instrument or agreed outcome with legal force."

"That is a very elastic phrase," Stern said.

Obama's climate team is reportedly working to put together a "politically binding" deal which would combine voluntary pledges with legally binding conditions from already existing treaties. Any such pact would avoid the need to seek ratification from the US Senate.

"Unfortunately, this would be just another of many examples of the Obama administration's tendency to abide by laws that it likes and to disregard laws it doesn't like -- and to ignore the elected representatives of the people when they don't agree," US Senate Republican Minority Leader Mitch McConnell said in response to reports outlining the administration's strategy.

Jennifer Morgan of the World Resources Institute said any international agreement faced a "very challenging road" in the United States.

"It's also a narrow road, because for most other countries around the world, having an agreement that is legally binding is a top priority," she said.

"They want to know that the US and other countries are going to implement their commitments. It will one of the big challenges of president Obama to navigate that with other countries."