Republican presidential candidate Donald Trump on Monday called on member countries of the North Atlantic Treaty Organization to spend at least 3 percent of their gross domestic product (GDP) on their defense, calling the current 2 percent guideline "the steal of the century."

The former president's remarks added to speculation that should he return to the White House, he could put pressure on South Korea to increase its financial contributions to the stationing of the 28,500-strong US Forces Korea.

"I'll insist that every NATO nation must spend at least 3 percent. You have to go up to 3 percent," he said in his address to the conference of the National Guard Association of the United States in Detroit, Michigan.

"Two percent is the steal of the century, especially when we're paying for it. You know, we pay for them. It's just not even believable," he added.

Trump claimed that for years, all NATO countries spent "far less" than 2 percent of GDP on their defense, which he said left American forces "overstretched."

"We were the ones making up the difference and paying for it to make up for shortfalls and help deter threats," he said.

In the wake of Russia's annexation of Crimea in 2014, NATO heads of state and government agreed to commit 2 percent of GDP towards defense spending to help ensure the transatlantic alliance's military readiness.

This year, 23 NATO allies are expected to meet or exceed the 2 percent target, compared to only three allies in 2014.

His remarks came as Seoul and Washington are set to hold their seventh round of negotiations in Seoul on Tuesday over the Special Measures Agreement, a deal that would determine South Korea's share of the cost for stationing USFK. The current SMA is set to expire at the end of next year.

The two sides launched negotiations in April apparently earlier than usual amid speculation that should former President Donald Trump return to the White House, he could play hardball in a way that could cause tension in the alliance.

Since 1991, Seoul has partially shared the cost for Korean USFK workers; the construction of military installations, such as barracks, as well as training, educational, operational and communications facilities; and other logistical support. (Yonhap)