Defense Secretary James Mattis said Tuesday he wants to see more evidence on how deep Russia’s support for the Taliban is because what he’s seen “doesn’t make sense.”

Asked by Rep. Niki Tsongas (D-Mass.) about Iran’s and Russia’s support for the Taliban, Mattis said that Iran is “doing what it usually does” in sowing regional chaos. But Russia, he said, has been harder to figure out.

“I want to see more evidence about how deep the support is,” Mattis said. “I need more definition on what is coming out of Russia. I can’t figure it out. It doesn’t make sense. But we’re looking at it very carefully.”

Mattis was testifying at the House Armed Services Committee alongside Joint Chiefs of Staff Chairman Gen. Joseph Dunford. It was the pair’s second hearing of the day on the U.S. strategy in Afghanistan.

Pentagon officials for months have been expressing concern about Russia’s forays into Afghanistan. They’ve also suggested, without explicitly confirming, that Russia has been arming the Taliban, which evolved from the mujahedeen the Soviet Union fought a war against in the 1980s.

In February, the top U.S. general in Afghanistan, Gen. John Nicholson, told senators the Russian support entailed trying to legitimize the Taliban by spreading a narrative that the Taliban is fighting the Afghan branch of the Islamic State in Iraq and Syria.

In April, Nicholson also said adamantly that he was “not refuting” reports that Russia is supplying weapons to the Taliban.

Speaking alongside Nicholson, Mattis said at the time that the United States would have to confront any such violation of international law.

Russia has repeatedly denied claims it’s supplying the Taliban, saying that such allegations are “fabrications” and “utterly false.”

On Tuesday, Mattis said it’s in neither Russia’s nor Iran’s interest to support the Taliban, which is what makes their support puzzling.

“Iran’s had their diplomats killed by Taliban,” Mattis said. “Russia certainly has had enough problems coming out of terrorism in south central Asia. So this doesn’t make sense, but then the world doesn’t always make sense. We’ll figure it out, and we’ll illuminate where it’s necessary in order to try to get a change in behavior.”

Dunford likewise said there is no “specificity” on Russia’s support for the Taliban.

“With the Russians, I don’t think we have specificity on support to the Taliban,” Dunford said.

But for Iran, he said, there are “no questions” of support.

“I think with regard to the Iranians there’s no question there’s a degree of support, as well as communications,” Dunford said.

Mattis also said Iranian support for the Taliban has always been “low level” and mostly financial.

“Out of Iran, it’s always been a low level of intermittent support for Taliban, mostly financial some weapons,” he said. “Iran is doing what it usually does in terms of trying to create chaos.”