By Andrew Gray

TAMPA, Fla., Oct 31 (Reuters) - U.S. Army Gen. David Petraeus took over the command running the wars in Iraq and Afghanistan on Friday and said it was vital to combine military and civilian power and work with allies.

Petraeus, hailed as an outstanding U.S. military leader for helping to pull Iraq back from the brink of all-out civil war, took charge of U.S. Central Command in a ceremony at the headquarters in Tampa, Florida.

Central Command, known as Centcom, oversees U.S. military operations and strategy in a volatile swathe of the world that covers 20 countries and stretches from Egypt across the Middle East and into south and central Asia.

Petraeus, 55, a fiercely competitive, media-friendly soldier who holds a doctorate in international relations from Princeton University, leaves on Saturday to visit the region.

His most pressing problems include rising insurgent violence in Afghanistan and safe havens for Taliban and al Qaeda militants across in border in Pakistan.

"From trans-national extremist organizations and industrial-strength insurgencies to weapons proliferation, a rise in piracy and persistent ethno-sectarian conflict, the Centcom area contains innumerable challenges," Petraeus said.

"Addressing these challenges requires comprehensive approaches that employ the whole of our government's capabilities in close coordination with host country and coalition governments and security forces," he said.

His remarks reflected Petraeus' belief that violence and instability cannot be solved by force alone and that economic development and political measures are also needed to address underlying causes.

Petraeus, 55, pursued that approach as the top commander in Iraq from February 2007 until last month and violence dropped to its lowest level in more than four years.

PRE-EMINENT SOLDIER

While other major factors also contributed to the security gains in Iraq and analysts argue about how durable they are, Petraeus' leadership has earned him widespread praise in the United States and beyond.

"He is the pre-eminent soldier-scholar-statesman of his generation and precisely the man we need at this command at this time," Defense Secretary Robert Gates said at the ceremony, conducted outdoors in bright sunshine.

"Under his leadership, our troops have dealt our enemies in Iraq a tremendous blow. Now he will take aim at our adversaries in Afghanistan."

One of Petraeus' main challenges will be to maintain his cautious approach to troop cuts in Iraq while meeting commanders' requests for more forces in Afghanistan, where insurgent violence has worsened this year.

That task may be more difficult if Democrat Barack Obama defeats Republican John McCain in next week's U.S. presidential election. Although he has reserved room for maneuver, Obama has suggested removing combat troops from Iraq in 16 months.

Petraeus took over at Central Command from Army Lt. Gen. Martin Dempsey, in temporary charge since Navy Adm. William Fallon quit in March after a magazine article said he was pushing President George W. Bush to avoid war with Iran. (Editing by David Storey)

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