BRUSSELS -- U.S. Defense Secretary Ash Carter travels to Brussels on Tuesday on a two-fold mission; to explore the future of NATO and increase contributions by allies -- as Russia keeps them nervous -- and the ongoing fight against ISIS.

On Wednesday, NATO Defense Ministers will hold a series of meetings that will come on the heels of the release of President Obama's budget, which will quadruple U.S. military spending in Europe. The priorities, according to one senior U.S. defense official, will be to fight ISIS and counter Russian aggression, among other threats.

A U.S. military trainer is seen at left as Ukrainian soldiers prepare for "live fire" exercises at a training center near Ukraine's border with Poland, late January, 2016. CBS/Cami McCormick

A second U.S. defense official said the focus will be readiness and the speed at which the alliance can move against those threats, particularly in Baltic countries.

"There will be some important changes for a NATO that is used to dealing with COIN (the military counterinsurgency strategy) and Afghanistan," he said. "NATO is trying to adapt itself."

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"The problems are coming at us quickly and with complexities," he said, adding that there is a difference in opinion over NATO's future among its members. Some are leaning towards more "coalitions of the willing," in the future, rather than relying on the entire international alliance.

"We feel strongly that NATO is an effective organization. But that doesn't mean NATO plays the same role it did in Afghanistan, but maybe (a role similar to) in Libya or Iraq. Libya started as a 'coalition' operation and was handed off to NATO," he said.

The spread of ISIS in Libya will also be a major point of discussion, but no major announcements are expected from the NATO meetings. Some NATO nations have made it clear they are not ready to commit to a military campaign in the country, which still lacks a unified central government in the chaos following dictator Muammar Qaddafi's toppling in 2011.

On Thursday, the focus will shift to battling ISIS in its heartland. At the first summit of its kind, Carter will present his strategy to partners and allies for the re-taking of Raqqa, the group's self-proclaimed capital in Syria, and Mosul, its stronghold in Iraq. The U.S. commander in charge of the anti-ISIS coalition's fight, Lt. Gen. Sean MacFarland, will also brief the summit.

There will be 28 nations in attendance, all of which are participating in some way in combat operations, including Iraq. Another 21 nations will be there in an observer capacity.

On the heels of Canada's announcement that it is ending its airstrikes but at the same time expanding its operations in areas such as Special Operations, other nations will be pushed to do more. The Netherlands and Saudi Arabia have offered to contribute more resources. Carter has repeatedly pushed Sunni Arab nations to get more involved.

According to The Associated Press, of the 10,060 airstrikes conducted over the past year and a half, U.S. warplanes have conducted all but 2,124 of them in Iraq and all but 208 in Syria.

In addition to more air power, the U.S. wants its partners to contribute more troops and trainers to the effort.

The summit comes at a potentially crucial time for the war in Syria. As CBS News correspondent Holly Williams reports, Bashar Assad's Syrian army -- with vital backing from the far-superior Russian air force -- is rapidly closing in on Aleppo, which ISIS has held significant parts of for years.

Thousands of residents of Aleppo, which was Syria's most populous city before the war started, have fled and massed at the Turkish border, desperate for refuge.

Many of the moderate rebel factions, including some which have received U.S. and coalition support, have been beaten back from villages and towns around Aleppo in recent weeks by the Syrian-Russian-Iranian alliance, and there are mounting fears the war could quickly tip in Assad's favor if the city falls back into regime hands.

The United Nations-backed Commission of Inquiry on Syria said in a report released Monday that so many thousands of people have been executed, beaten to death or otherwise left to die by the Assad regime during five years of war that it likely amounted to "extermination" under international law.

While the regime, the Islamic extremist groups and other rebel factions have repeatedly been accused of human rights violations and war crimes during the war by the U.N. and other groups, Monday's report was the first time the global body had raised the possibility of eventual extermination charges.

Filed by CBS Radio News correspondent Cami McCormick.