* About 30 nations to meet March 31 climate deadline

* Mexico is first emerging economy to submit plan

By Alister Doyle and Valerie Volcovici

OSLO/WASHINGTON, March 29 - The United States will submit plans for slowing global warming to the United Nations early this week but most governments will miss an informal March 31 deadline, complicating work on a global climate deal due in December.

The U.S. submission, on Monday or Tuesday according to a White House official, adds to national strategies beyond 2020 already presented by the 28-nation European Union, Mexico, Switzerland and Norway.

Together, they account for about a third of world greenhouse emissions. But other emitters such as China, India, Russia, Brazil, Canada and Australia say they are waiting until closer to a Paris summit in December, meant to agree a global deal.

"It's not the ideal situation," said Niklas Hoehne, founding partner of the New Climate Institute in Germany which tracks submissions, known as Intended Nationally Determined Contributions (INDCs).

In 2013, the United Nations invited INDCs by March 31, 2015, from governments "ready to do so" - the early, informal deadline was meant to give time to compare pledges and toughen weak ones.

Late submissions complicate the Paris summit because it will be far harder to judge late INDCs.

"The earlier the better," said Jake Schmidt, of the U.S. National Resources Defense Council. "It allows people to look at each others' targets and judge whether or not they pass muster."

The White House official noted that both the United States and China already outlined plans last year, saying: "That adds up to a fantastic running start."

The United States plans to cut emissions by 26 to 28 percent below 2005 levels by 2025.

Mexico on Friday became the first emerging economy to make a pledge, saying its emissions would peak by 2026. Developing countries set less strict goals than developed states since they need to burn more energy to grow their economies.

Mexico's plan "certainly should create incentives for developed countries to come in," said Jennifer Morgan, of the World Resources Institute, noting rich nations are meant to lead.

The U.N. Climate Change Secretariat will compile by Nov. 1 submissions made by October 1.

It says it is already clear that INDCs will fall short of emissions cuts needed to limit temperatures to 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times, a U.N. ceiling to avert floods, desertification, and rising seas.

"We expect many more countries to submit INDCs over the coming days, weeks and months," said Nick Nuttall, spokesman for the Secretariat. (Reporting By Alister Doyle and Valerie Volcovici; editing by William Hardy)