James Schlesinger, a tenacious, blunt-talking former Pentagon chief who served as a cabinet officer under three US presidents, died Thursday. He was 85.

A Harvard-educated economist who became a powerful figure in the 1970s, as well as an influential nuclear strategist, Schlesinger served as defense secretary under Richard Nixon and Gerald Ford and was named the country's first energy secretary under Jimmy Carter.

"Secretary Schlesinger was a brilliant economist and had a keen understanding of defense budgeting, our country's nuclear enterprise, and our most advanced weapons programs," Pentagon chief Chuck Hagel said.

The Center for Strategic and International Studies, a Washington think tank where Schlesinger served as a trustee, confirmed his death.

Schlesinger died at the John Hopkins Bayview Medical Center in Baltimore from complications of pneumonia, The New York Times reported, citing his daughter Ann.

Schlesinger served as defense secretary from 1973 to 1975, a time of upheaval with US military morale on the decline and the White House preoccupied by the Watergate scandal.

A Republican with hawkish tendencies, Schlesinger clashed with Congress over defense spending and argued for bigger budgets to counter the Soviets. He developed a nuclear strategy that called for retaliatory strikes against Soviet military targets but not population centers.

During the peak of the Watergate scandal in August 1974, Schlesinger became so concerned about Nixon's emotional state that he ordered the military not to respond to White House orders unless cleared by him and secretary of state Henry Kissinger.

He confirmed the instructions years later, which also included plans to deploy troops in the US capital in the event of any turmoil surrounding Nixon's political succession.

Ford retained him as defense secretary but he was dismissed in November 1975, after repeatedly clashing with the president and lawmakers.

Before the Pentagon, he served as chairman of the Atomic Energy Commission, which promoted nuclear energy. And in 1973, Nixon named him director of the Central Intelligence Agency after the president fired spy chief Richard Helms for refusing to obstruct the Watergate investigation.

In a mere five-month stint at the CIA, Schlesinger learned that intelligence officers who were barred by law from spying on Americans had carried out break-ins in the United States on orders from the White House. He sacked 1,000 employees of the 17,000-strong work force and ordered an internal investigation into past operations.

After his cabinet positions, he later served on numerous advisory boards, including one for the Pentagon in 2008 looking at problems in the Air Force's management of the country's nuclear forces.