The United States led calls at UN climate talks Saturday for efforts to curb methane emissions but also pushed a deeply controversial drive to boost nuclear energy to curb global warming.

With smoggy skies in Dubai highlighting the challenges facing the world, other pledges are expected at the COP28 conference, including stepping up the deployment of renewable energy.

The use of nuclear power as a cleaner alternative to fossil fuels is highly controversial as environmental groups are concerned about safety and the disposal of nuclear waste.

But more than 20 nations ranging from the US to Ghana, Japan and several European countries said in a declaration that it plays a "key role" in the goal of achieving carbon neutrality by mid-century.

They called for the tripling of nuclear energy capacity by 2050 from 2020 levels.

"We are not making the argument to anybody that this is absolutely going to be a sweeping alternative to every other energy source," US climate envoy John Kerry said at COP28.

"But we know because (of) the science and the reality of facts... that you can't get to net zero 2050 without some nuclear," he said.

The other signatories include Britain, France, South Korea, Ukraine and the United Arab Emirates. But nuclear powers Russia and China did not sign up.

Environmental group said the 2011 Fukushima nuclear disaster in Japan highlighted the dangers of atomic power.

"While we appreciate that the Biden administration is looking to invest in alternatives to fossil fuels, we don't have time to waste on dangerous distractions like nuclear energy," said its North American director Jeff Ordower.

Experts point to the fact that nuclear plants can take decades to go into service.

"Nuclear energy takes much longer than renewable energy to be operational," added.

Fossil fuel expansion 'frightening' 

The declaration came as more than 50 world leaders took the stage at COP28 for the second day in a row, though US President Joe Biden and Chinese leader Xi Jinping are skipping the talks.

US Vice President Kamala Harris announced a $3 billion contribution to a global fund to help developing countries with the energy transition and the effects of climate change -- its first pledge to it since 2014.

"Today, we are demonstrating through action how the world can and must meet this crisis," Harris said.

German Chancellor Olaf Scholz said energy transition "has to be now", adding: "We all have to demonstrate the same determination to phase out fossil fuels, beginning with coal."

Meanwhile Colombia became one of the largest fossil fuel producers to join a group of climate-vulnerable island nations calling to end new development of planet-heating coal, oil and gas.

Colombia said its decision to join the Fossil Fuel Non-Proliferation Treaty Initiative was an important step in its climate plans.

Colombian Environment Minister Susana Muhamad said it was "frightening" that governments continued to plan to expand their fossil fuel exploitation.

While nations are locked in contentious negotiations on a phase out or phase down of fossil fuels, there is broad backing for the tripling of renewable energy by 2030, an issue that will feature highly on Saturday.

The US and China, the world's two biggest emitters of greenhouse gases, will later hold a summit with the UAE on methane emissions.

"The science must be simple: to turn down the heat, you simply have to turn down the methane," said Barbados Prime Minister Mia Mottley.

Washington has announced it will tighten curbs on methane emissions from the oil and gas industry.

The new standards would phase in eliminating routine flaring of natural gas produced by oil wells and require comprehensive monitoring of methane leaks from wells and compression stations.

Methane emissions also come from the agriculture sector, with cows and sheep releasing the gas during digestion and in their manure.

Methane "is the most destructive gas", Kerry said.