UNITED NATIONS (Reuters) - U.N. member states owe nearly $2 billion in peacekeeping funds, United Nations chief Antonio Guterres has warned, with the United States responsible for more than a third of that.

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“Active peacekeeping missions are soon expected to face liquidity gaps due to late payments and increasing arrears,” Guterres wrote in a Jan. 11 letter to the 193 member states. “Arrears are nearing $2 billion and are likely to keep growing.”

Guterres also said that while 152 members had paid in full what they owed for a separate U.N. regular budget in 2018 - a record - more than $528 million was still outstanding.

The United States is responsible for 22 percent of the $5.4 billion regular budget for 2018 and 2019 and more than 28 percent of the $6.7 billion peacekeeping budget for the year to June 30.

The United States owed $381 million to the regular budget as of Jan. 1 and some $776 million to the peacekeeping budget, U.N. officials said, figures the U.S. mission to the U.N. confirmed.

President Donald Trump says Washington is shouldering an unfair burden of the cost of the United Nations and pushed for the world body to reform operations.

In December, the U.N. General Assembly agreed what percentage of the regular and peacekeeping budgets each country would pay for the next three years.

The U.S. contribution to the regular budget is already limited to 22 percent and Washington unsuccessfully pushed to cap its peacekeeping contribution at 25 percent - as required by U.S. law - instead of more than 28 percent.

“The lack of agreement on a 25 percent ceiling will cause the organization to continue to face a three percent shortfall in its peacekeeping budget,” Cherith Norman Chalet, the U.S. envoy for U.N. Management and Reform, told the U.N. budget committee Dec. 22.

This amounts to some $200 million, diplomats said. The United Nations has 14 peacekeeping operations, half of them in Africa.

“Current cash balances cover less than two months of operations, compared to four months last year,” Guterres said of peacekeeping finances in his letter. Last year he twice wrote to warn states about the “troubling financial situation”.

India’s U.N. Ambassador Syed Akbaruddin said the $2 billion shortfall was “unsustainable.”

Some troop contributing countries “are owed amounts equivalent to their annual assessed contributions for 100 years. Some even more,” he tweeted on Wednesday.

The top contributing countries are Ethiopia, India, Bangladesh, Nepal and Rwanda. They pay their troops according to their national salary scales and are reimbursed by the U.N. As of July 2018, the U.N. paid $1,428 a month per soldier.

The United Nations says its peacekeeping operations cost less than half of 1 percent of world military expenditures.