(CNN) Former Secretary of State Rex Tillerson said Monday that "clearly asking for personal favors and using United States assets as collateral is wrong," a swipe at President Donald Trump as the investigation into his dealings with Ukraine intensifies.

PBS Newshour's Judy Woodruff asked Tillerson at an event in Texas what he would have done if -- while he was secretary of state -- US military assistance to Ukraine had been made conditional on the launching of a political investigation, which is what House Democrats accuse Trump of having done.

"I'd rather not answer a hypothetical on something that's so visible in the hearings today," he said initially.

When Woodruff pressed him on it, he replied, "Clearly, asking for personal favors and using United States assets as collateral is wrong."

"There's just no two ways about it -- so if you're seeking some kind of personal gain and you're using whether it's American foreign aid or American weapons or American influence, that's wrong," Tillerson continued. "And I think everyone understands that."

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