Rex Tillerson broke with President-elect Donald Trump, who nominated him for secretary of state, on whether the US would fulfill its commitments to defend its NATO allies.

Trump suggested during his campaign that, as president, he would defend NATO allies only “if they fulfill their obligations” to the US. He has also discussed the need for US allies to contribute more to their own defense.

Sen. Rob Portman questioned Tillerson, the former Exxon Mobil CEO, on this issue during his Senate confirmation hearing Wednesday.

“That security umbrella we’ve provided since World War II has kept the peace,” Portman said.

He continued: “Just to be clear, because I know there was a discussion about NATO earlier, particularly about Article 5, which talks about ‘an armed attack against one or more members shall be considered an attack against them all.’ Can you just clarify that you believe Article 5 creates a binding obligation to assist any member of the alliance who is a victim of aggression regardless of their size or geographic location?”

Tillerson responded, "Yes, sir, I do."

Portman then asked Tillerson if he would, as secretary of state, "ever threaten to break the US commitment to Article 5 as a means of pressuring allies to spend more on defense."

Tillerson said he would "not recommend that."

Earlier during Tillerson's hearing, Sen. Todd Young asked if the US should defend a NATO member country from Russian invasion.

In response, Tillerson called the Article 5 commitment "inviolable" and said the US would stand by it.