A top NATO official has conceded that the alliance would have to settle for "less of the pie" in the future due to conflicting spending priorities for its members.

Jamie Shea, deputy assistant secretary general for emerging security challenges, also says a "lack of ships" is a growing problem for NATO members.

NATO wants countries to spend at least 2 percent of their gross domestic product (GDP) on defense needs, as the alliance requires. But, according to NATO, only five of its members out of 28, including the UK, met the goal of spending 2 percent of GDP on defense in 2015.

NATO also wants alliance members to commit 20 percent of defense budgets to research and development.

Speaking at an event in Brussels on Thursday, Shea admitted that, with alliance members increasingly focusing on a range of other nondefense priorities, NATO may have to accept "less of the pie" in future.

The Briton cited one example being the decision of the Belgian government, following the Islamic State attacks on Brussels on March 22 March, to deploy more than 1,000 troops to the city's streets for surveillance purposes.

The Belgian army has also recently been called to help out in prisons in Brussels and Wallonia because of a strike by prison guards.

Migrants and refugees arrive in the first international-standard refugee camp on March 7, 2016 in Grande-Synthe, northern France. France opened its first international-standard refugee camp in the teeth of official opposition on March 7 while demolition continued at the nearby Calais
Migrants and refugees arrive in the first international-standard refugee camp on March 7, 2016 in Grande-Synthe, northern France. France opened its first international-standard refugee camp in the teeth of official opposition on March 7 while demolition continued at the nearby Calais "Jungle" camp. The new camp, featuring some 200 heated wooden cabins and proper toilets and showers, has been built by Doctors Without Borders (MSF) with the support of the local town hall, despite opposition from the French government. / AFP / FRANCOIS LO PRESTI (Photo credit should read FRANCOIS LO PRESTI/AFP/Getty Images)

Migrants and refugees arrive in the first international-standard refugee camp on March 7 in Grande-Synthe, northern France. A NATO official says the migrant crisis is one of the issues competing for member nations' funding.

Photo Credit: Francois Lo Presti/AFP/Getty Images

Shea said NATO members face a range of what he called "nontraditional" security and other threats and challenges, such as climate change and the ongoing refugee crisis in Europe, that are eating into national budgets.

Apart from its involvement in such areas, NATO, he said, had "lots of other demands" on its forces, including "hard-end" military operations.

While NATO has a limited role in crises such as the migrant emergency facing Europe, he said the alliance would seek to "play its full part."

Shea, who has held several posts with NATO since 1980, also expressed concern about a "lack of ships" for alliance members.

"Even the US recently had to ask France to provide aircraft in the East Mediterranean Sea to help during an anti-Islamic State operation," he said.

"If that can happen to the Americans, think of the strain on European defense forces," he said.

Shea was speaking at a conference on EU security, migration and borders.