By Alister Doyle

MARRAKESH, Morocco (Reuters) - There is "no turning back" from a global accord to combat climate change, Morocco's foreign minister said on the eve of U.N. talks in Marrakesh amid fears that Donald Trump will try to pull out if he wins the U.S. presidency.

The number of countries that have ratified the 2015 Paris Agreement to slow global warming by shifting from fossil fuels has risen to 100 nations in recent days, including OPEC's top oil producer Saudi Arabia, the United Nations said.

The U.S. election is casting a shadow over the meeting of officials from almost 200 nations in Marrakesh from Nov. 7-18 to work on ways to implement pledges made in Paris to adopt cleaner energies such as wind and solar power.

Trump wants to ditch the Paris Agreement, doubting mainstream scientific findings that droughts, floods and rising sea levels are linked to human activities. By contrast, his Democratic rival Hillary Clinton is a supporter.

"There is no possible turning back in the negotiation on what was agreed in Paris," Moroccan Foreign Minister Salaheddine Mezouar, who will preside at the talks, told a news conference.

"We can only advance," he said, without naming Trump.

He said the agreement had strong momentum after formally entering into force on Friday with backing from major emitters led by China, the United States and India.

Asked about Trump's threats, U.N. climate chief Patricia Espinosa noted that any state wanting to pull out would have to follow formal legal procedures, due to last four years.

"The agreement has entered into force and we are all obliged to deliver on those commitments," she said. Still, the agreement lets all nations to define their own commitments for fighting climate change and has no sanctions for non-compliance.

With the U.N. accord legally in place, Trump may find it easier if he wins to oppose U.S. President Barack Obama's plans to cut U.S. greenhouse gas emissions by 26 to 28 percent cut by 2025 from 2005 levels.

Trump says the agreement would undermine U.S. jobs, such as in the coal industry.

The Morocco meeting is due to start writing a detailed "rule book" for the Paris Agreement, likely to take two years, and find ways to act such as raising finance to help developing nations cut their rising emissions.

The Paris Agreement entered into force on Friday after it passed a twin threshold last month of support from 55 nations representing 55 percent of global emissions.

(Editing by Raissa Kasolowsky)