* Defense budgets becoming tighter but threats remain
* NATO must be more flexible in order to respond quickly
* Pentagon chief urges alliance to join U.S. strategic shift
By David Alexander
LONDON, Jan 18 (Reuters) - U.S. Defense Secretary Leon Panetta called on Friday for NATO to reinvent itself as a more agile alliance with a broader outlook embracing the Asia-Pacific and able to respond to new threats from Islamic militancy.
Panetta said as the alliance winds down the Afghanistan war and cuts defense spending to fit shrinking budgets, it would still face challenges from Islamist militants as well as countries like Iran and North Korea.
"NATO can no longer be an alliance focused on a single type of mission, whether deterring the aggression of another superpower or conducting stability operations like Afghanistan," Panetta told a group at King's College in London, in one of his last major speeches as defense secretary.
"To be prepared to quickly respond to a wider range of threats in an era of fiscal constraint, we must build an innovative, flexible, and rotational model for forward-deployed presence and training," he said.
Panetta, whose trip to Europe this week has coincided with Islamist militant violence in Mali and Algeria, is due to leave office in the next few weeks once his successor is confirmed by the U.S. Congress.
His comments echoed those 18 months ago by his predecessor Robert Gates, who used his last speech in Europe as defense secretary to bluntly warn that NATO risked irrelevance because members were unwilling to invest in defense, saying that Washington could not shoulder the entire financial burden.
Panetta noted that some European allies had been concerned about the new U.S. defense strategy, which calls for a rebalance toward the Asia-Pacific region.
To make that shift under tight fiscal constraints - defense spending will be cut by $487 billion over a decade - the Pentagon has cut U.S. forces in Europe, causing economic pain in some areas.
Panetta said that even as it cut back some forces, the United States was supporting regular rotational deployments to conduct joint training with the allies and ensure the sides are able to operate together on future missions.
"Global security is not a zero-sum game, and neither are the security commitments of the United States," Panetta said. "More importantly, Europe's economic and security future is, much like the United States', increasingly tied to Asia."
"The bottom line is that Europe should not fear our rebalance to Asia, Europe should join it," he said. (Reporting By David Alexander)