The Iraq and Afghanistan wars will cost the United States between $4-6 trillion in the long term, constraining the government's budget for decades to come, a study said.

Harvard University scholar Linda Bilmes concluded that the United States will face increasing costs to care for an estimated 2.5 million veterans, and to pay down debt incurred by borrowing to pay for the wars.

"As a consequence of these wartime spending choices, the United States will face constraints in funding investments in personnel and diplomacy, research and development and new military initiatives," said the report released Thursday.

"In short, there will be no peace dividend, and the legacy of Iraq and Afghanistan wars will be costs that persist for decades," it said.

Bilmes, who served in government under former president Bill Clinton, calculated that the United States has already spent nearly $2 trillion directly for the two wars launched by former president George W. Bush.

But Bilmes' study said the biggest cost would be medical care and disability benefits, saying that more than half of the 1.56 million troops discharged from service have already been granted benefits for life.

Bilmes, who called the numbers unprecedented, said that costs will climb over decades. She wrote that the peak year for disability payments over World War I, which ended in 1918, was 1969 as veterans became elderly.

"The magnitude of future expenditures will be even higher for the current conflicts, which have been characterized by much higher survival rates, more generous benefits and new, expensive medical treatments," she said.

Bilmes also factored in debt, finding no precedent for a time when the United States went to war while lowering taxes, with the possible exception of the Revolutionary War when US colonies borrowed from France.

The study also looked at social costs, with families burdened with the effects of the deaths or injuries of service members.

The United States is expected to maintain a limited military presence in Afghanistan after 2014, when President Barack Obama plans to withdraw combat troops first sent after the September 11, 2001 attacks.

Opinion polls show that most of the US public has grown weary of the longest US war, in Afghanistan, and is critical of Bush's decision to invade Iraq a decade ago.

Donald Rumsfeld, the defense secretary under Bush, said before the invasion that the Iraq war would cost around $50 billion and called higher estimates "baloney."