Cuts to the UK's defence spending are a concern, Nato Secretary General Anders Fogh Rasmussen has said.
He told BBC 1's Politics Show the cuts were a "matter for concern but I'm convinced that the UK will fully live up to her commitments in our alliance".
Asked whether he hoped spending would not be reduced further, Mr Rasmussen said he did not want to "interfere" in domestic political decisions.
Defence spending in the UK is expected to fall by 8% over four years.
In last month's Spending Review, David Cameron announced Harrier jump jets, the Navy's flagship HMS Ark Royal and planned Nimrod spy planes would be axed.
Some 42,000 Ministry of Defence and armed forces jobs would also be cut by 2015, he said.
However the government has committed to meet Nato's target of spending 2% of GDP on defence.
Mr Rasmussen told the programme: "As secretary general of Nato, I would urge allies to co-operate, to pool resources together, to go for collective solutions, because we could achieve economies of scale and make more efficient use of our resources if we pool resources together."
Last week, Britain and France signed an agreement to work together on defence and nuclear programmes.
Mr Rasmussen also said the next Nato summit in Lisbon would set out a plan for handing over control of Afghanistan to domestic security forces.
The summit is expected to announce the transition from the international coalition to Afghan forces would begin early next year, with full responsibility being given to the Afghans by the end of 2014.
"This is a time perspective which of course must be conditions-based - it can't be calendar-driven, we have to make sure that the Afghans are really capable to take responsibility before we hand over responsibility to Afghan security forces - but I'm quite confident that this timetable is realistic," Mr Rasmussen said.
He added that Mr Cameron's timetable to withdraw UK combat troops by 2015 fitted "very well" with Nato's plans.
He also said could not rule out Nato intervening in states where terrorists were operating.
"We have no plans whatsoever to expand our activities, our hands are full in Afghanistan. But the examples you mention - Yemen, Somalia - just underline how important it is that we finish our job in Afghanistan successfully," he said.
"I would not exclude the possibility that we can get engaged in coming years if we see failed states being a potential threat because they offer a safe haven for terrorists."