NATO Secretary-General Jens Stoltenberg said Tuesday a meeting of the alliance’s defense ministers will focus on the need for members to increase the amount of money they spend on defense.

He told reporters in Brussels ahead of the meeting that countries boosted defense spending by $10 billion last year, but that only five of the 28 NATO members are meeting the target of allocating 2 percent of their GDP to defense.

U.S. President Donald Trump criticized NATO members during his campaign, saying not enough were paying their fair share. At one point he suggested those who did not share their part of the burden would not be automatically defended by the U.S.

Since taking office, Trump has expressed his support for NATO, including, Stoltenberg said, in two phone calls with him.

“President Trump has in both the phone calls also underlined the importance of fair burden sharing, and that those countries that spend less than 2 percent have to meet the 2 percent target. And I agree with him,” Stoltenberg said.

New U.S. Defense Secretary Jim Mattis is traveling Tuesday to Brussels to join the NATO talks.

Stoltenberg said the alliance is looking forward to “be able to sit down with him and to discuss many different topics, including burden sharing, but also NATO’s role in fighting terrorism.”

The NATO chief noted that some members are struggling with economic challenges, but he said the goal is to begin to with stopping cuts in defense spending and eventually shift to meeting the full target amount.

NATO heads of state are due to meet later this year, and Stoltenberg said exactly what kind of language or proposals may emerge in order to help boost defense spending remain to be seen from that meeting and the one this week.

Stoltenberg also said he was looking forward to an upcoming meeting with Russian Foreign Minister Sergei Lavrov, and that NATO needs to be both firm in presenting a credible defense while also keeping diplomatic lines open.

“Especially in difficult times as these we need open political dialogue with Russia,” he said.