Lima climate talks at crossroads on ways to fighting warming.............

* U.N. talks on final day, amid fears of weak ambition

* 190 countries split on scope of pledges to slow warming

* Lima deal could point to aspirations for Paris summit in 2015

By Alister Doyle and Valerie Volcovici

LIMA, Dec 12 (Reuters) - About 190 nations were at a crossroads about how boldly to combat global warming on the final day of United Nations talks in Lima on Friday amid fears that low ambition could undermine a U.N. climate summit in Paris next year.

The Dec. 1-12 talks, which opened with hopes for new momentum after a climate deal between China and the United States last month, focused on the scope of pledges that countries are due to make early next year to tackle warming.

Those national pledges, due by an informal deadline of March 31, 2015, will be the building blocks of a global deal to be agreed in Paris in December 2015 and meant as a step toward reversing rising world greenhouse gas emissions.

Options discussed in Lima range from obliging nations to publish a vague outline of their carbon plans on a U.N. website to making all provide detailed projections in tonnes of greenhouse gas emissions that will be reviewed by experts.

"There's the good, the bad and the ugly," said Alden Meyer, of the Union of Concerned Scientists, who said "ugly" would mean only vague action and "good" would be detailed accounts. Delegates say the talks may well run overnight into Saturday.

Under the deal announced with the United States, China has promised its emissions will peak around 2030, for instance, but may not give the exact numbers and opposes the idea of a review by other countries.

The United States favors a review but is not insisting that countries should be willing to toughen their plans if challenged. The European Union and many developing nations want detailed accounting by all and a strong review.

"We have agreed to find the balance between the several options," German Environment Minister Barbara Hendriks said.

This year is set to be the warmest on record, and scenarios by a U.N. panel of scientists indicate that the world should get on track to slash emissions to a net zero before 2100.

Marlene Moses of Nauru, chair of the Alliance of Small Island States, which fears rising sea levels, criticized China's reluctance to give full information.

She said that China was saying: "'We'll show you our cards but don't read them'. ... We are being asked to sign on to an agreement that puts us underwater. That's not fair to us."

Samantha Smith of the WWF conservation group said there was a rising risk of only weak action since the Paris accord is due to enter into force only from 2020.

"Leaders who are in office today won't be accountable," she said.