By James Mackenzie
ROME (Reuters) - U.S. President Barack Obama visited the Colosseum in the latest stop on his European tour on Thursday, calling the ancient Roman arena "remarkable, unbelievable" and he was almost as lavish in his praise for Italy's 39-year-old Prime Minister Matteo Renzi.
Renzi, Italy's youngest ever prime minister, has set a testing agenda of economic and constitutional reforms to help the eurozone's third-largest economy recover from its longest recession since World War Two.
"I want to say that I've just been impressed by the energy and vision that Matteo's bringing to his position," Obama said during an hour-long joint news conference in Rome.
"There's a seriousness and ambition of ideas, and I think the spirit and the energy of the Italian people has the opportunity to be unleashed in a way that will be good for Italy, but it will also be good for Europe," he said.
"So it's wonderful to see this new generation of leadership coming to the fore."
The comments were a welcome boost to Renzi, who took over leadership of Italy's cross-party ruling coalition after a party coup last month, pledging to push forward more aggressively on reforms than his predecessor Enrico Letta.
Over the past two weeks, Renzi has met French President Francois Hollande and German Chancellor Angela Merkel to seek their backing for a more ambitious push to lift the economy out of the doldrums.
With memories still fresh of Italy's central role in the eurozone crisis which took the single currency to the brink of collapse in 2011, leaders around the world have been watching Renzi closely as he has launched into his agenda.
Breaking occasionally into English and making frequent references to the importance of the U.S. alliance and the sacrifices of American soldiers during World War Two, Renzi could scarcely conceal how pleased he was by the visit and pledged to continue with his programme of change.
"Italy has no alibi," he said. "Italy cannot think that its own problems and its own opportunities can come from the outside, from Europe or the United States. We have to change ourselves," he said.
The mutual congratulations concealed more delicate issues, notably over Italy's reconsideration of its planned order for 90 F-35 combat jets, which Renzi's center-left Democratic Party wants to cut "significantly" to ease the pressure on its strained public finances.
The ambitious timetable Renzi has set himself for the coming three months has also run into some headwinds in parliament which are likely to grow as his campaign against the entrenched privileges of the political class continue.
Obama avoided directly commenting on the F-35 issue but said that despite the need to make defence spending around the world more efficient, America's NATO allies had to keep defence budgets on an upward trajectory with "everybody pitching in".
He contrasted the U.S. record of spending 3 percent of its economic output on defence with Europe, which spent 1 percent.
There was also no word on the backroom manoeuvres currently taking place to choose a new secretary general of NATO. Former Norwegian Prime Minister Jens Stoltenberg has been seen as front-runner with the backing of the United States but former Italian Foreign Minister Franco Frattini is also in the race.
(Additional reporting by Steve Scherer, Jeff Mason, Roberto Landucci and Steve Holland, Editing by Angus MacSwan)