Sen. Marco Rubio challenged Donald Trump’s pick for Secretary of State Rex Tillerson for his views on Vladimir Putin, demanding to know whether he thought the Russian president was a war criminal.

“I would not use that term,” Tillerson replied.

Rubio listed public reports of Putin-linked bombings and attacks on civilians as well as public reports of civilian dissidents, political opponents and journalists dying under mysterious circumstances.

“I would want to have much more information before reaching a conclusion,” Tillerson replied, calling Rubio’s concerns “very serious charges.”

Rubio specifically referred to Russia’s actions in Syria, describing Putin’s actions as “war crimes.”

“I find it discouraging your inability to cite that, which I think is globally accepted,” Rubio replied.

Tillerson said he would like more detailed information about Russia’s action before advising the president.

“I look forward if confirmed to becoming fully informed, but I am not willing to make conclusions on what’s only publicly available,” Tillerson replied.

“None of this is classified, Mr. Tillerson,” Rubio replied. “These people are dead.”

Tillerson shot back that he didn’t dispute that the people were dead, but wanted more information before advising the president’s reaction.

Rubio also challenged Tillerson’s response calling economic sanctions of countries “troubling,” after he also said he would need more information before advising the president on such issues.

Rubio’s aggressive stance was not unexpected after he signaled in December that he was concerned about Tillerson’s closeness with Putin, while the nominee did business with Russia as the CEO of ExxonMobil.

Republicans and Democrats are likely to use the Tillerson hearings to highlight their differences with President-elect Trump on Russia.