Forbes estimated David Koch wealth at the end of his life at $42.4 billion. (AFP)

David Koch, a billionaire American libertarian and influential donor to conservative causes, has died at age 79 after a long battle with cancer, his brother Charles said Friday.

Koch retired last year as executive vice president of Koch Industries, the conglomerate he co-owned with his older brother and built into the second largest family-owned company in the United States.

"We wish for all to celebrate the life and impact of this most generous and kind man," Charles Koch said in a statement.

"He believed he had a responsibility to a world that had given him so many opportunities to succeed."

Both brothers were a force behind the scenes in Republican politics, donating heavily to candidates and causes that reflected their conservative economic positions.

But David was socially liberal -- a supporter of abortion rights and same-sex marriage as well as a non-interventionist foreign policy.

Both brothers were recognized in 2015 for bipartisan work on prison reform in the United States.

Among those offering condolences was Secretary of State Mike Pompeo who, like the Koch brothers, is from Kansas.

David Koch ran as the Libertarian Party's vice presidential candidate in 1980, but later broke with it and swung to the Republicans.

He joined his brother Charles in financing a network of conservative organizations, in particular Americans for Prosperity, centered on demands for low taxes and deregulation with the aim of influencing US elections.

Those organizations helped fuel the rise of the Tea Party movement in 2010, mounting a frontal Republican challenge to former Democratic president Barack Obama.

The Kochs' power was such that in the next presidential elections, Republican candidates vied for their endorsement, attending exclusive conferences the brothers organized.

Donald Trump, whose conservative credentials the brothers viewed with suspicion and who in turn often mocked them, was the exception.

Of the two, Charles has been most involved in the political networks while David had been more active in philanthropic endeavors, mainly in New York.

He was remembered as an important financial contributor to cultural organizations and medical research.

He was diagnosed with advanced prostate cancer 27 years ago but through "a combination of brilliant doctors, state-of-the-art medications and his own stubbornness kept the cancer at bay," Charles said.

Born in Wichita, Kansas, Koch studied chemical engineering at the Massachusetts Institute of Technology before joining Koch Industries.

Forbes estimated his wealth at the end of his life at $42.4 billion, making him one of the world's richest people.