President Trump will push North Atlantic Treaty Organization allies to meet their defense spending obligations on Thursday when he attends his first NATO summit in Brussels, Secretary of State Rex Tillerson said on Wednesday.

"As the president has said, he really wants participating NATO members to step up and fully meet their obligations of the burden sharing," Tillerson told reporters aboard Air Force One. "Two percent of GDP was a target they all agreed to. I think you can expect the president to be very tough on them, saying: ‘Look, the U.S. is spending 4 percent. We're doing a lot. The American people are doing a lot for your security for our joint security. You need to make sure you're doing your share for your own security as well.'"

"So I think that's going to be the core of his message to NATO," he said.

Tillerson said the U.S. would like to see NATO allies become more engaged in the global coalition to fight the Islamic State, but he noted the issue may not be resolved at the NATO summit scheduled for Thursday.

"NATO joining the ISIS coalition, we do think that would be a really important step for them to take. They've been an observer," Tillerson said. "But they've become more and more engaged in the actual fight to defeat ISIS."

The secretary of state said Trump has yet to decide whether to send additional U.S. forces to Afghanistan and whether to pull the U.S. out of the Paris climate accords.

Trump "hasn't made a final decision" on the fate of the climate pact, Tillerson said, and will likely delay one until "after we get home."

"The Afghan policy review is still a work in progress," Tillerson said. "It's probably a couple of weeks away at least before we're going to be ready to present something to the president."

Trump has been an outspoken critic of many NATO allies' inability or unwillingness to meet the minimum requirements for military spending under the treaty. The president has even referred to NATO as "obsolete," a comment that could complicate his first appearance at a meeting of the alliance.