The government of Kuwait owes some of the United States’ top hospitals nearly $700 million in unpaid bills for sophisticated medical care provided to Kuwaiti nationals, a debt the frustrated institutions have spent years trying to collect.

The 45 medical centers — which include Massachusetts General Hospital in Boston, MD Anderson in Houston and others — have worked unsuccessfully on their own and through the State Department to recoup payment for specialized cancer, heart, pediatric and other care provided to thousands of people from the Persian Gulf nation, one of the wealthiest in the world.

In June, the facilities banded together in an unprecedented group effort to seek repayment of the $677.4 million debt. In July, they enlisted the aid of a congressman to insert a “Sense of Congress” amendment seeking Kuwaiti repayment into defense authorization legislation.

“U.S. hospitals have provided care in good faith to your government based on your government’s guarantee of payment,” the U.S. Cooperative for International Patient Programs wrote to the Kuwaiti ministers of health, finance and foreign affairs on June 11. “Our members expect payment of these bills in good faith as well.”

Noting the financial damage to hospitals wrought by the covid-19 pandemic, the organization wrote that “with . . . individual hospitals having to bear the burden of maintaining multimillion-dollar balances over multiple years, the current system has become impractical and unsustainable for hospitals across the United States.”

Kuwaiti officials did not respond to calls and an email to the country’s Washington embassy this week.

The State Department said in a statement that “it has raised this issue with the Kuwaiti government and is working to find a resolution.”

It is unclear why Kuwait — a staunch U.S. ally of about 1.4 million citizens and 3 million expatriates, controlled by the ruling al-Sabah family — would fall so far behind on its bills. Several hospital officials, speaking on the condition of anonymity because of the delicate nature of their effort, said they had become accustomed to payment delays from Kuwait, possibly because of bureaucracy or an antiquated system for paying overseas obligations. The country pays only for its own citizens.

Still, the current backlog has lasted much longer than any previous one, said Jarrett Fowler, director of the international patient cooperative, who wrote the letter to Kuwaiti officials. Previous debts have been outstanding for as long as a year, he said. Kuwait is alone among nations who send patients to U.S. hospitals in its extended failure to pay.

“Me writing that letter is evidence that we’re in extraordinary circumstances,” Fowler said in an interview. “I’ve never done anything like this before.” He said the Kuwaiti debt “continues to rise as they continue to not pay the bill. . . . We currently don’t have any indication from them as to when it will be paid.”

Fowler’s letter called the $677 million “a conservative estimate” of the amount owed. It may not include doctors’ fees or ancillary services for some medical centers. He listed more than $183,000 owed from 2017; $172.8 million from 2018; $414.9 million from 2019; and $89.4 million so far in 2020.

The effort to recoup the money is led by Massachusetts hospitals, which are owed nearly a quarter of the total, Fowler said.

As oil prices fell in recent years, Kuwait saw its first budget deficit in a decade in 2015 and announced cuts to fuel subsidies in 2016, according to the CIA’s World Factbook. In January, the country announced it expected a budget deficit of more than $30 billion for the 2020-2021 fiscal year.

But Kuwait continues to pour billions into a sovereign wealth fund that Rep. Jim McGovern (D-Mass.), who is aiding the hospitals’ effort, told Congress is the fifth-largest in the world at $592 billion. In his remarks to Congress, McGovern said, “the Kuwaitis say this matter is pending in their parliament.”

One expert consulted by The Washington Post said the debt could just be the result of an administrative problem.

Unpaid bills have plagued U.S. patients and hospitals for years as the cost of medical care here has soared. The pandemic has made that situation much more critical for hospitals. With patient volume down and lucrative elective procedures canceled or postponed, U.S. hospitals and health systems could lose $323.1 billion in 2020, according to the American Hospital Association.

The mounting Kuwaiti debt, though small in comparison, is a problem for some hospitals. “It does affect, overall, our financial outlook and our ability to plan,” said one official from a major hospital, who spoke on the condition of anonymity because of the negotiations.

Foreign nationals represent a relatively small income stream for elite academic medical centers that provide specialized care that may not be available in other countries. That can include proton beam and immunotherapy for cancer, organ transplants or advanced diagnostics and treatment for people with multiple underlying conditions, experts said.

A 2017-2018 survey conducted by the U.S. patient cooperative showed that 57,190 foreign patients produced about $2 billion in gross revenue for the 50 medical organizations that reported their data.

Kuwait sent the fourth-largest number of adult patients to the United States, after Canada, China and Mexico, and the tiny nation’s citizens received more inpatient care than people from any other country. Kuwait sent the second-largest number of pediatric patients, behind the United Arab Emirates.

According to a report in the Arab Times, Kuwait authorized 3,900 visits for overseas care in the first half of 2018.

McGovern said it is time for the Kuwaitis to pay their bills, or face increased pressure from the United States.

“There’s a thousand different excuses out there and none of it is resulting in our hospitals getting paid,” he said. “Friends do not treat friends like this. All we’re saying is, ‘please pay and if not, we will look at further legislation that quite frankly won’t be as polite as the Sense of Congress resolution.’ ”