The Pentagon has been caught flat-footed after President Trump's meeting with Russian President Vladimir Putin this week, with defense officials struggling to explain statements coming out of Moscow that the two leaders reached agreements involving military issues.

Adding to the angst, Defense Secretary James Mattis has kept a low profile amid backlash to Trump's comments in Europe. While he met with his counterparts there, Mattis remained offstage for the president's trip and was not in attendance at this week's Cabinet meeting.

Mattis, a retired Marine Corps general and former NATO commander, has made no secret of his opinion of Russia, telling reporters traveling with him last week that Moscow has chosen to "undermine the fabric of nations that are young in their democratic processes ... whether through false news reporting, economic strictures and interventions."

"They are not seen as helpful, would be probably the most polite way to describe it," he said.

U.S. law also prohibits military-to-military cooperation between the United States and Russia. Congress passed the measure in the wake of Russia's 2014 annexation of Crimea and it has been renewed every year since.

But Mattis's response to various declarations by Trump in Europe - including over defense spending and potential areas of cooperation with Russia - has put a spotlight on issues where the Defense chief's positions may contrast the prevailing views in the administration.

"The Pentagon now is left holding the bag on how they fulfill the either promises or vague responses on [military-to-military] cooperation, and whether those issues are in the national security interest of the United States," said Mark Simakovsky, a nonresident senior fellow at the Atlantic Council.

"I do think on Russia and on NATO, Secretary Mattis has a different view than the president and I think [it] highlights a great source of potential tension as the president will want his Cabinet secretaries to support normalizing relations with Russia."

The Defense chief was out of the spotlight for much of Trump's recent weeklong trip to Europe, during which the president chided NATO allies over defense spending, criticized British Prime Minister Theresa May's handling of her country's exit from the European Union and appeared to side with Putin's denial of Russian meddling in the 2016 presidential election.

Mattis traveled with Trump to the NATO summit in Belgium but chose to work behind the scenes. He later told reporters that the "hearty discussion" in Brussels was "very helpful" but did not mention Trump by name.

In Croatia last week, where he met with defense leaders at the U.S.-Adriatic Charter (A5) meeting in Zagreb, Mattis also quoted Vice President Pence on the importance of America's continued commitment to Western Balkan nations.

"I'm here today to reinforce America's commitment to the security and stability of southeast Europe. As Vice President Pence said during last year's A5 summit, the Adriatic Charter is a testament to the U.S. commitment to a Europe that is whole, free, and at peace," Mattis said.

Mattis reiterated the United States's commitment to NATO the following day when he met with Norwegian Foreign Affairs Minister Ine Eriksen Søreide, referring to "shared and vital interests across the North Atlantic transatlantic community, from Canada to the United States to Europe."

Simakovsky said Mattis's choice to visit the Nordic and Baltic countries after the NATO summit was part of a "reassurance tour" that started even before the Brussels meeting.

"Pointedly, he didn't mention Trump," Simakovsky said of Mattis's remarks on the post-NATO trip.

Mattis himself has largely dismissed speculation that Pentagon officials were doing damage control with allies following the NATO summit, calling such reports "fascinating."

"I love reading fiction so it was stimulating to read it," he told reporters. "I must tell you - it must have been the most pleasant damage control in the A5 I ever could have imagined with the level of unity of purpose that we experienced there."

James Carafano, a defense policy expert at the Heritage Foundation and member of the Trump transition team, argued that Mattis seems genuinely pleased with the results of the NATO summit.

"There's a whole range of issues that they wanted work on, command and control, mobility, the Iraq mission, Ukraine and all that happened," Carafano told The Hill on Friday.

"Mattis, his job is to be the secretary of Defense and to deal with the substance of that. I really do think that's what he really focuses on, and so from that perspective he was really pleased."

But Mattis hasn't been so vocal on news coming out of the Trump-Putin summit.

Trump's schedule on Thursday listed a closed-door meeting with Mattis, but the Defense secretary has not appeared in public this week nor commented publicly on the president's meeting with Putin in Helsinki.

The silence also spread to the Pentagon, where press officials this week were unable to answer questions on Trump and Putin's discussion and potential agreements made between the two leaders that might affect the military.

U.S. Central Command head Gen. Joseph Votel told reporters Thursday that he had received no new guidance on changes toward the Russian military in Syria since the Helsinki summit.

"For us right now it's kind of steady as she goes," Votel said.

"We have received no further direction than we've currently been operating under."

Even administration officials are working to determine what agreements, if any, were made between Trump and Putin during their more than two-hour private meeting, which was quickly shadowed by Trump's performance during Monday's joint news conference.

Carafano argued that Mattis has simply been busy and doesn't want to get sucked into the partisan debate about Trump's style.

"He's not a politician. It's his job to run the Pentagon so I don't think he sees a productive role for him to be in the middle of people debating about the president's negotiation style," he said. "But he's been very, very vocal talking to allies about the substantive issues which is where he kind of sees his role."

Russia, meanwhile, has touted the Trump-Putin summit as a success and said "verbal agreements" were reached regarding increased cooperation on international security matters such as Ukraine and Syria.

"The Russian Defense Ministry is ready for practical implementation of the agreements in the sphere of international security reached by Russian and U.S. Presidents, Vladimir Putin and Donald Trump, at their Monday's summit in Helsinki," Igor Konashenkov, a spokesman for the ministry, said.

The statement from Moscow is unsurprising, Simakovsky said, as it seeks to shape the narrative and quickly advance their objectives following the summit.

But Simakovsky, who worked in the Office of the Secretary of Defense for Policy during the Obama administration, said he expects Pentagon officials reacted to Russian statements on agreements made in Helsinki with a mix of "surprise," "concern" and "restraint."

"I'm sure there were several meetings that took place that essentially described how they would proceed but also out of an abundance of caution both because they don't know what was promised but also because I think there will be a skeptical view of the value of reinvigorating mil-to-mil engagement and even cooperation on a range of issues," he said.

Despite the Pentagon's skepticism at Russian engagement, Simakovsky added, Trump's invitation for Putin to visit the White House in the fall could force the department's hand.

"That's why Putin's coming here and that's why the president wants him here is because he knows those meetings are forcing functions for the U.S. government, because they're going to have to come up with an agenda and he's going to put pressure on his Cabinet agencies to come up with concrete measures that will make it appear the U.S. and Russia are finding ways to normalize and cooperate," Simakovsky said.