BRUSSELS – US Secretary of Defense Ash Carter left a three-day trip to NATO with new commitments to help in the fight against the Islamic State group, but questions remains about how successful the campaign can be if Russia remains active in Syria.

Carter traveled to Brussels for a two-part business trip. First, he took part in the NATO ministerial, with a major focus on fiscal reassurance for European partners from the Pentagon's budget request. But the centerpiece of the event was day two, when Carter hosted a meeting of 27 other defense ministers whose nations participate in the coalition against the terrorist group, commonly known as ISIS or ISIL.

The big fish for Carter: a commitment from Saudi Arabia to increase its kinetic airstrikes inside Syria, as well as consideration of Saudi troops on the ground, potentially giving the Sunni ground force publicly called for by President Barack Obama.

In addition to Saudi Arabia, three countries — Poland, Romania and Denmark — have stepped up their commitments this week, either for kinetic strikes or training missions, a senior defense official said.

On Feb. 12, the United Arab Emirates also pledged to restart participation in the air campaign against ISIL, Carter said. On the same day, diplomats in Germany — including top American and Russian officials — announced the groundwork for a potential ceasefire in Syria's five-year civil war.

Carter also highlighted a "significant development" — that NATO Secretary General Jens Stoltenberg and he are exploring the possibility that NATO could join the anti-ISIS coalition as an active participant.

"NATO as a new member would bring unique capabilities that could be brought to bear against ISIL, including experience in building partner capacity, training ground forces and providing stabilization support."

Leading up to the meeting, Pentagon officials, including Carter, emphasized that US is looking for countries to contribute in ways beyond military assistance, such as training police or providing funding for the Iraqi government. The latter is increasingly important, with the US State Department estimating Iraq faces a monthly financing gap of $5.3 billion.

Emma Ashford, an analyst with the Washington-based CATO Institute, says emphasizing non-military contributions is an easy way to increase participation from partner nations.

"It's easier for most states to commit to this kind of help, which doesn't require parliamentary approval or even much popular support," she said. "And some aspects of the non-military campaign, like humanitarian aid and help with counterterrorist financing, are really important aspects of the overall strategy."

But Ashford also raises concerns that countries may now have less incentive to contribute militarily.

"Carter may say he wants to 'accelerate' the campaign against ISIS, but it's not clear how to do that without more actual military contributions on the ground from regional governments," she said.

The Russian Challenge

According to the senior defense official, the partners in Brussels were all on board with the strategic plan outlined by the US, with one caveat – concerns over Russia's continued presence in Syria.

"One thing that was striking was criticism of Russia. Several ministers said the coalition need to be more vocal in its criticism of Russia," the official said. "They all acknowledged it's a very complicated situation in Syria, that arguably there has been more progress in Iraq, and Syria will remain arguably the more challenge of the two. But the criticism of Russia was widespread and that was a consistent theme."

Evelyn Farkus, who served as deputy assistant secretary of defense for Russia, Ukraine and Eurasia from 2012 until November, agrees Russia remains a huge challenge to solving the Syrian situation.

Ahead of Carter's trip, Farkus advocated leveraging a "whole of government approach" toward the Kremlin, including increasing already-imposed sanctions on the government of Russian President Vladimir Putin.

"In context of the Syria problem, we could actually think about applying sanctions to Russia that we would coordinate with our Middle Eastern allies and Europe," she said. "It's something that should be seriously considered."

"We're not going to convince Russia if they're winning on the battlefield," she added. "So what can we do to change the dynamic? Maybe we can change what's happening on the battlefield, but in addition we should be considering other things we can do to increase our leverage vis-a-vis Russia.

Carter, for his part, said Russia is contributing to a "very cruel movement of a large number of people in Syria," a movement that has forced NATO to step its maritime commitment.

During the NATO ministerial, Stoltenberg pledged Standing Maritime Group 2 to the Aegean Sea to monitor the flow of migrants. The group would primarily conduct surveillance of refugees and alert local authorities to their location, although Supreme Allied Commander Europe Gen. Philip Breedlove indicated the mission parameters could potentially change in the future.

Stoltenberg also announced that the more capability would be needed for Standing Maritime Group 2 to carry out the operations. Breedlove confirmed that "several" nations have agreed to add ships, but would not identify which ones.

Twitter: @AaronMehta