(This version of the Sept. 16 story corrects "Nigerian forces" to "Nigerian authorities" in paragraph 6)

By Helen Reid

LONDON (Reuters) - Nine million people urgently need aid in northeast Nigeria and nearby countries, as political violence and mass migration have caused a humanitarian emergency, a United Nations official said on Friday.

Islamist group Boko Haram has waged an insurgency since 2009, displacing 2.1 million people and killing thousands, a crisis worsened by a "demographic explosion" in the Lake Chad basin by migrants making their way toward Europe, U.N. regional humanitarian coordinator Toby Lanzer said.

"With population growth of that speed and nature, in an area where everyone is already poor, the environment is incredibly stressed, and there is a never-ending stream of heavier violence, it is only natural to conclude that more people will migrate," Lanzer said at the London think-tank Chatham House.

U.N. and non-governmental organizations need $559 million from September to December to ease the crisis in the Lake Chad nations of Nigeria, Niger, Cameroon and Chad where more than 6 million people are "severely food insecure" and 568,000 children acutely malnourished, he said.

The United Nations has appealed to Britain and other Western governments for help and heads of state of the Lake Chad basin and donor countries will meet on the margins of the U.N. General Assembly on Sept. 23, he added.

"Nigerian authorities are really trying their best but they have to be supported in their efforts, otherwise we will not meet the needs of the population," said Mercedes Tatay, of Médecins sans Frontières, which has been working in Maiduguri, the main city in northeast Nigeria, since 2014.

The International Committee of the Red Cross operation in the Lake Chad neighboring countries is the second biggest after its Syria operation, a spokesman said, its budget rising from 40 million pounds ($52 million) in 2015 to 105 million pounds in 2016.

Nigeria, Africa's largest economy, entered recession at the end of August. "The government's capacity to run itself is very stretched," said Lanzer.

($1 = 0.7653 pounds)

(Editing by Robin Pomeroy)