U.S. military forces in the Middle East nearly bombed a group of Russian mercenaries working with pro-government forces in Syria for a second time, Defense Secretary Jim Mattis revealed Tuesday.

Mattis, speaking to reporters at the Pentagon, also defended the mass expulsion of Russian intelligence officers around the world in response to Moscow's use of a deadly nerve agent in attempting to assassinate a defector in Britain.

On the Syrian incident, Mattis said within the past week a group of Russian mercenaries moved across a river that was a line set up to deconflict U.S.-backed and Russian-backed rebel groups and into an area were Russians were not supposed to be operating.

"These were forces moving into more advanced positions, too close," he said.

After discussions between Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, and Gen. Valery Gerasimov, chief of the general staff, the Russian mercenary forces pulled back.

"So it looks like, this time, it was resolved through the deconfliction communication line," Mattis said. "It did not go into harm's way as it did there a month ago."

As many as 200 Russian mercenaries in Syria were killed Feb. 7 during a large-scale U.S. airstrike near the Euphrates River in Syria.

The mercenaries were part of a covert private military army known as the Wagner Group that is backed by the Kremlin.

Mattis said he could not answer questions about the February attack on why the Russians moved on "our forces at Deir ez-Zor."

"I still cannot answer that question," he said.

Deconfliction talks with Russian officers, apparently at a lower-ranking level, told the Pentagon the Russians "weren't their forces," he said.

"I have no evidence that they were being dishonest and that they knew in fact, these forces were theirs," Mattis said. "Them taking our forces under fire, obviously, they paid a very heavy price for that."

In the recent incident, Russian forces again moved across the Euphrates into a no-go zone.

"We think that the potential for a clash there has, thanks to the Russian direction to this group, has been reduced," he said.

Asked if the pullback after the Dunford-Gerasmov phone call shows that the Russians are under Moscow's control, Mattis said: "I believe they are."

A defense official said the Russian mercenaries and pro-Syrian government forces were spotted last week building defensive positions near U.S.-backed forces.

"At a certain point, it looked threatening," the official said, noting a buildup of earthen berms and position of weapons pointed at the U.S.-backed forces as if in preparation for an attack.

That prompted the call from Dunford to Gerasimov on Feb. 21, leading the Russian and Syrian forces to move away from the positions and back across the river.

The mercenaries killed in February were part of special force of deniable Russian forces that have been dubbed "little green men" for their green military uniforms worn without any identifying insignia.

On the mass expulsions of Russians, Mattis said the actions were a justified response to the first use of a nerve agent in Europe since World War II.

The latest expulsions took place in Brussels Tuesday when the NATO alliance expelled seven diplomats from Russia's mission there.

NATO Secretary-General Jens Stoltenberg said Tuesday that NATO is also cutting the maximum size of the Russian mission from 30 people to 20 people.

"It sends a very clear message to Russia that it has costs," Stoltenberg said, referring to the March 4 attack in Salisbury, Britain, against former Russian double agent Sergei Skripal and his daughter Yulia Skripal.

Both remain in critical condition after a military nerve agent called Novichok was used on them. A British policeman also was stricken by the nerve agent after treating the Skripals.

Mattis said the action by the 29-nation alliance was an important show of NATO solidarity.

The attack in Salisbury was a "reckless activity" by the Russian government.

Asked if the chemical attack was an act of war, Mattis said, "I think they're doing things that they believe are deniable."

"They take insignia off soldiers' uniforms and they go into Crimea," he said. "They say they have nothing to do with what's going on with the separatists in eastern Ukraine; I'm not sure how they can say that with a straight face. They point out that it can't be proven who had tried to kill the person in Salisbury."

"They're doing things they believe are deniable. And so they're trying to break the unity of the Western alliance, NATO, and that sort of thing," he said.

He declined to speculate whether Russian leader Vladimir Putin was behind the nerve agent attack.

"Certainly he's responsible as the head of state," he said.

The retired Marine four-star general also said the way to counter Putin's regime and activities is for the United States act in concert with democracies.

"We have to show where all of these democracies stand together," he said. "Notice what is emblematic of each of the countries that's evicting Russian diplomats right now. I notice the one common thread. Some have a queen, some have a president—they're all democracies. And I think that's the statement about how you get to them."

The United States expelled 60 Russians and closed the Russian consulate in Seattle. Britain expelled 23 Russians and 24 other European states and Australia also ordered out Russian officials in an operation aimed at diminishing Russian intelligence operations.

Russia conducted spying and assassination operations through the FSB Federal Security Service, SVR civilian spy service, and GRU military service.

The attack in Salisbury is believed by U.S. intelligence to have been a GRU operation.

Vasyl Hrytsak, head of Security Service of Ukraine, known as SBU, told the Washington Free Beacon recently that the Wagner group is a "private army" that was set up, armed, and funded by Russian intelligence services for military, terrorist, and other criminal activities in support of Russian geopolitical interests.

On suspicious packages sent recently to military and intelligence facilities in the Washington area, Mattis said all the packages are under control of law enforcement agencies and no one was injured.

The packages, some containing what appeared to be bomb components, were found at the White House, FBI, CIA, and several other government facilities, along with the Army's Fort McNair in southwest D.C. and Fort Belvoir in Virginia.

The FBI arrested Thanh Cong Phan, a mentally disturbed man in connection with the mailed packages, in Everett, Washington.

Mattis also said he planned to meet soon with incoming White House National Security Adviser John Bolton.

"We're going to sit down together, and I look forward to working with him," he said. "No reservations, no concerns at all. Last time I checked, he's an American and I can work with an American."

Asked how he plans to work with Bolton who holds more conservative views, Mattis said: "Well, I hope that there's some different worldviews. That's the normal thing you want, unless you want groupthink."

On Yemen, Mattis said the Pentagon is helping Saudi Arabia with missile defenses to counter missile attacks fired by Iranian-backed Houthi rebels.

"They don't intercept them if they hit the open desert; they don't waste a missile or an intercept on it," he said.

"But the Iranian support to the Houthis that gives them this capability is clearly being countered thanks in part to the American systems that they bought and, more importantly right now, to the advisers we brought in to assist them in defense of the kingdom of Saudi Arabia."

A recent missile strike landed in the Saudi capital of Riyadh for the first time.

"I think it's very serious," Mattis said. "I think Iran should reconsider sending missiles down there."

Mattis declined to answer questions about the recent policy announcement that the U.S. military will block most transgenders suffering from gender dysphoria from serving in the military.

"I really don't want to get into that one issue right now because it's in litigation," he said.

Advocacy groups are suing the military over the issue in a bid to force them to allow transgenders in the ranks.

Defense officials said the policy was aimed in part at denying transgenders who join the military in order have the Pentagon pay for sex-change surgery and treatment.

Some transgender troops currently in the armed services will be allowed to remain.

However, a new Pentagon policy require all troops to be capable of being deployed on missions will likely result in many transgender military personnel being expelled.