The United States is helping lead the charge at UN climate talks to phase out fossil fuels, but the superpower's status as the world's top oil producer is already casting doubt on its credibility.

Often cast by environmentalists as a laggard or even villain in climate diplomacy, especially under Republican presidents, the United States has startled some veteran observers at the COP28 summit by supporting calls to end extraction of oil, gas and coal, with major oil exporter Saudi Arabia the most vocal opponent.

The cooperative approach marks a new legacy-making effort by US climate envoy John Kerry, a former secretary of state, senator and presidential contender who turns 80 on Monday in the heat of negotiations.

Recalling his role in the 2015 Paris accord, which set an ambition of capping global warming at 1.5 degrees Celsius above pre-industrial levels, Kerry said he felt a sense of "mission and urgency" in Dubai following a slew of record temperatures and disasters.

US President Joe Biden has poured money into climate projects, with his signature legislative package, the $1.2 trillion Inflation Reduction Act, funding major investment in electric cars, power grid upgrades and other green areas.

But the United States, after years of expansion driven by new technologies, is also by far the world's largest oil producer, pumping last year an average of 20 million barrels a day, or 21 percent of the global total, according to the US Energy Information Administration.

"We ourselves are drunk on oil and natural gas that the United States exports every single day," Senator Ed Markey, a member of Biden's Democratic Party elected to Kerry's former seat, told reporters at COP28.

"Our companies don't want to stop doing that either," he said, warning the United States was in little position to "teach temperance".

- Drilling at home -

Biden has kept up the pace of his climate-sceptic predecessor Donald Trump in approving drilling on public lands, in part citing a need to make up for shortfalls after Western sanctions imposed on Russia over its invasion of Ukraine.

"While it's talking about a phase-out here, we also need the Biden administration to do more at home," said Allie Rosenbluth of the advocacy group Oil Change International.

She also said the administration was betting on "technologies that we know don't work and that are prohibitively expensive" -- a reference to the heavy investment in carbon capture, which aims to reduce emissions from fossil fuels.

Draft texts at COP28 talk of reducing "unabated" emissions from fossil fuels -- code for still allowing oil, gas and coal but trying to reduce their impact.

Still, some environmentalists were pleasantly surprised that the United States appeared to be going along with strong language on ending fossil fuel extraction.

"There is a tangible shift that we've seen in the US foreign policy on climate where they are moving away from reliance on carbon capture and storage and other technologies and pure market mechanisms to address the climate emergency," said Jean Su of the Center for Biological Diversity.

- Eyes on Trump -

There is one elephant in the room at COP28 -- US elections are less than a year away, with Trump seeking to return to the White House.

Senator Lisa Murkowski, a moderate Republican from oil- and gas-producing Alaska, acknowledged the climate effects of fossil fuels but questioned, especially in remote areas, "this approach that we can just keep it in the ground."

"To move to a phase-out, I think, does not recognise the transition reality that we are currently facing," said Murkowski, the only Republican in the US Senate delegation at COP28.

But Trump and many other hardline Republicans take a more confrontational approach and deny the scientific consensus on climate change.

Kerry argued that presidential leadership will matter less as US companies and state and local governments had already committed to go green.

"There was a time when that might have made all the difference, but not now," Kerry said.

"Even when Donald Trump was president, 75 percent of the new electricity in the United States came from renewables. I assume he didn't know it -- he would have tried to stop it."

Many Republicans also adamantly oppose climate assistance -- a key part of UN-backed talks with wealthy countries promising to help hardest-hit nations.

Vice President Kamala Harris, visiting COP28, promised $3 billion to a Green Climate Fund.

Democratic Senator Chris Coons, however, voiced optimism on aid and noted that many of Biden's climate investments are going to parts of the United States that usually vote Republican.

"Am I suggesting that were the former president to be our next president that everything would be fine? Not at all," Coons said.

"But I am saying that there is broad enough and deep enough support for continuing investments to combat climate change."