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 two per cent of their gross domestic product (GDP) on defence needs as the alliance requires. But, according to Nato, only five of its members out of 28, including the UK, met that goal in 2015.
Nato also wants alliance members to commit to 20 per cent of defence spending on research and development.
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Speaking at an event in Brussels last Thursday (May 26), Shea admitted that, with alliance members increasingly focusing on a range of other non-defence spending priorities, Nato may have to accept "less of the pie" in future.
He cited one example being the Belgian government's decision, following the terrorist attacks on Brussels on 22 March, to deploy more than 1000 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 of prison guards.
Shea said Nato members faced a range of what he called "non-traditional" security and other threats and challenges, such as climate change and the ongoing refugee crisis in Europe, which was 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 had 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 concerns about a "lack of ships" for alliance members.
He said, "Even the US recently had to ask France to provide aircraft in the east Mediterranean sea to help during an anti-Islamic State operation.
"If that can happen to America think of the strain on European defence forces," he said.
Shea was speaking at a conference on EU security, migration and borders.