BAKU (Reuters) - The U.N.'s climate chief called on leaders of the world's biggest economies on Saturday to send a signal of support for global climate finance efforts when they meet in Rio de Janeiro next week.

The plea, made in a letter to G20 leaders from UN Framework Convention on Climate Change Executive Secretary Simon Stiell, comes as negotiators at the COP29 conference in Baku struggle in their negotiations for a deal intended to scale up money to address the worsening impacts of global warming.

"Next week's summit must send crystal clear global signals," Stiell said in the letter. 

He said the signal should support an increase in grants and loans, along with debt relief, so vulnerable countries "are not hamstrung by debt servicing costs that make bolder climate actions all but impossible".

Business leaders echoed Stiell's plea, saying they were concerned about the "lack of progress and focus in Baku". 

"We call on governments, led by the G20, to meet the moment and deliver the policies for an accelerated shift from fossil fuels to a clean energy future, to unlock the essential private sector investment needed," said a coalition of business groups, including the We Mean Business Coalition, United Nations Global Compact and the Brazilian Council for Sustainable Development, in a separate letter. 

Success at this year's U.N. climate summit hinges on whether countries can agree on a new finance target for richer countries, development lenders and the private sector to deliver each year. Developing countries need at least $1 trillion annually by the end of the decade to cope with climate change, economists told the U.N. talks.

But negotiators have made slow progress midway through the two-week conference. A draft text of the deal, which earlier this week was 33-pages long and comprised of dozens of wide-ranging options, had been pared down to 25 pages as of Saturday.

Sweden's climate envoy, Mattias Frumerie, told Reuters the finance negotiations had not yet cracked the toughest issues: how big the target should be, or which countries should pay.

"The divisions we saw coming into the meeting are still there, which leaves quite a lot of work for ministers next week," he told Reuters.

European negotiators have said large oil-producing nations including Saudi Arabia are also blocking discussions on how to take forward last year's COP28 summit deal to transition the world away from fossil fuels.

Saudi Arabia's government did not immediately respond to a request for comment.

Progress on this issue has been dire so far, one European negotiator told Reuters.

Uganda's energy minister, Ruth Nankabirwa, said her country's priority was to leave COP29 with a deal on affordable financing for clean energy projects. 

"When you look around and you don't have the money, then we keep wondering whether we will ever walk the journey of a real energy transition," she told Reuters.   

(Reporting by Kate Abnett, Valerie Volcovici, Simon Jessop; Editing by Frances Kerry)