Two U.S. congressmen have sent a letter to State Secretary Mike Pompeo and Defense Secretary Mark Esper, urging them not to ask Seoul to "exponentially increase" its annual contribution to the upkeep of American forces here.

The letter came amid U.S. President Donald Trump's continued pressure on South Korea to agree to a five-fold hike. It was sent by Eliot Engel, the chairman of the House Foreign Affairs Committee, and Adam Smith, the chairman of the House Armed Services Committee, both of whom are opposition Democrats.

"We agree that our allies and partners should fairly contribute to the cost of our presence overseas," the letter says. "But at a time when the United States, South Korea and Japan should be working jointly to counter regional security threats ranging from increased North Korean provocations to growing Chinese assertiveness across the region, U.S. demands for a massive increase in South Korean annual contributions serves as a needless wedge between us and allies."

But at the same time Trump was also putting pressure on NATO member countries and Japan to pay more. At a gaffe-ridden NATO summit in London, he said he was asking five more countries besides Korea to increase their share of defense costs.

"I've asked Japan. I said to Prime Minister Abe, a friend of mine, Shinzo. I said, 'You have to help us out here. We're paying a lot of money. You're a wealthy nation. And we're, you know, paying for your military, essentially," he claimed Tuesday.

Trump also asked NATO member states to increase their defense spending to four percent of their GDP.

A new category of the cost for "the operations and readiness posture of U.S. Forces Korea" has been created to hike Korea's putative defense bill, a senior official in the Trump administration said.

Under the current Special Measures Agreement, Korea is only to contribute to the salary for Korean employees of the USFK, and costs for military construction and logistics support.

