She told BBC Radio 4`s Today programme that cuts to defence budgets in Europe were "concerning".
Though Britain currently meets the Nato defence spending target, there is no commitment to honour it beyond the general election.
The ambassador said she had flown to Brussels to encourage the leaders of the Nato countries to meet the defence spending target of 2% of GDP.
Ms Power said that in "most cases" defence spending in Europe was shrinking, despite a growth in defence threats.
`Awful lot of threats`
She warned: "The number of missions that require advanced militaries to contribute around the world is growing not shrinking".
The UK government is committed to the 2% spending target until the end of this Parliament, but there has been no commitment beyond that from either the Conservatives or Labour.
Britain is one of the few countries to meet the target, Ms Power said.
She added: "The United Kingdom is exceptional - has been a staunch Nato ally and has stepped up - whether on Isil (Islamic State) or on Ebola or in Afghanistan".
But the ambassador warned: "We`re looking at an awful lot of threats and many of those threats are migrating into Europe, at least in the form of Isil, and yet we haven`t seen European defence spending reach the 2% of GDP level that European leaders committed to."
UK cuts `inevitable`
On Monday the Royal United Services Institute (Rusi) said it was inevitable that the UK`s defence spending would drop below the 2% target.
The defence think tank warned that up to 30,000 more military personnel could lose their jobs by 2020, whatever the outcome of the general election.
The warning followed concerns from the head of the US Army over the impact of spending cuts on the UK`s armed forces.
Chief of Staff General Raymond Odierno voiced his reservations about the falling proportion of the UK`s national wealth being spent on the military.
Despite these concerns, Ms Power told the Today programme: "This is not about one country, but it is about a larger, collective challenge that we are now facing."
She said a small investment into UN peacekeeping could make a "tremendous difference".
The ambassador appealed to governments for aid in the form of military sophistication, niche capabilities, attack helicopters and intelligence.
"Those kind of capabilities could strengthen the missions as a whole and thereby diminish the threat that all of us face," she added.
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