German Foreign Minister Sigmar Gabriel told reporters in Estonia on Wednesday that NATO had not agreed on every nation in the alliance having to meet a rigid target of spending 2 percent of its gross domestic product on defense.

"I have a concern that politicians make public promises that they can't fulfill later on," Gabriel said. He said that NATO had merely agreed that member nations would make efforts to reach the 2 percent target.

"There is no apodictic 2 percent goal but rather...we should be moving in that direction," he said at the start of a visit to Baltic states. Germany now spends 1.2 percent of GDP on defense.

At a NATO summit in Wales in 2014, allies agreed to end years of defense cuts that left Europeans without vital capabilities. They agreed all allies should reach the target of spending 2 percent of economic output on defense every year by 2024, although the goal is not legally binding.

The issue has become a source of tension between Chancellor Angela Merkel's Christian Democrats and Gabriel's Social Democrats, junior partners in the ruling coalition government, who are hoping to unseat Merkel in September national elections.

Defence Minister Ursula von der Leyen, a conservative, has criticized Gabriel for his remarks at the Munich Security Conference, where he warned against focusing solely on defense spending and unleashing a new arms race.

Merkel has said Germany needed to fulfill its commitment to boost defense spending to meet NATO's target of 2 percent.

Germany has come under increased pressure since the election of U.S. President Donald Trump to meet NATO's defense spending target.