LONDON/WASHINGTON, Nov 4 (Reuters) - A landmark global accord to combat climate change officially entered into force on Friday, putting pressure on countries to start executing plans to slash greenhouse gas emissions to limit global warming.

The 2015 Paris Agreement was agreed last December by almost 200 countries and has been described as the most complex global treaty since the Marrakesh (trade) Agreement, signed in 1994.

The Paris accord passed a threshold on Oct. 5 of 55 nations accounting for more than 55 percent of greenhouse gas emissions, allowing it to come into force 30 days later.

Ratification was swift compared to other international treaties, showing strong international support, but around 100 countries have yet to ratify it.

“This is a moment to celebrate. It is also a moment to look ahead with sober assessment and renewed will over the task ahead,” United Nations’ climate chief Patricia Espinosa said in a statement.

“In a short time - and certainly in the next 15 years - we need to see unprecedented reductions in greenhouse gas emissions and unequaled efforts to build societies that can resist rising climate impacts,” she said.

The Paris Agreement seeks to wean the world economy off fossil fuels in the second half of the century, limiting the rise in average world temperatures to “well below” 2.0 degrees Celsius (3.6 Fahrenheit) above preindustrial times.

It also notes an ambition to limiting temperature rise even further to 1.5 degrees.

The pact kicked off a rolling start in the Pacific region on Thursday, home to low-lying islands states which are in danger of rising sea levels.

On the same day, the annual report of U.N. Environment analyzed countries’ current pledges for emission cuts and said they were not sufficient.

Even if emission-cutting pledges under the Paris agreement are fully implemented, predicted 2030 emissions could put the world on track for a temperature rise of 2.9 to 3.4 degrees Celsius this century, the report said.

The latest round of U.N. climate talks begin on Monday in Marrakesh, Morocco, where representatives from countries will try to find ways to implement the agreement and work out the rules.

Environmental campaign groups, as well as some businesses, investors and academics said the meeting in Marrakesh must keep up the spirit of international support for climate action.

“Even with the commitments made in Paris and encouraging action on the ground, we will not meet our aspiration of limiting warming to 1.5 degrees unless we move faster and at the scale that is needed,” World Bank Group President Jim Yong Kim said.

“As the world heads into (the meeting) in Marrakesh, we must regain the sense of urgency we felt a year ago,” he said.