By Randall Mikkelsen

ORLANDO, Florida (Reuters) - Sen. Barack Obama got his first intelligence briefing as the Democratic U.S. presidential candidate this week and spy agencies are ready to do the same for Republican rival Sen. John McCain, a senior U.S. official said on Thursday.

Thomas Fingar, deputy director of national intelligence for analysis, said U.S. spy agencies had also begun reanalyzing and updating reports from around the world in preparation for the next president, who will take office in January.

"We've begun to engage with the campaigns," Fingar told an intelligence conference in Florida. "Sen. Obama received a briefing on Tuesday." Intelligence officials would not discuss what topics were covered in Obama's briefing.

The briefing was given in Chicago by Director of National Intelligence Michael McConnell -- who normally briefs President George W. Bush six days a week -- and other senior officials, an official familiar with the process said.

McCain was to formally accept the Republican Party nomination for the November 4 election on Thursday evening.

Obama's briefing was probably tailored to his requests, the official said. Fingar said briefings would be "substantive," but it would be inappropriate for a candidate to seek a copy of Bush's. The eventual president-elect will receive similar briefings, he told reporters later.

Each candidate can receive any information given the other, Fingar said. As senators, McCain or Obama could confidentially request information, but as candidates, any such request can be shared with the opponent, Fingar said.

"Our approach in this is complete transparency." he said.

National security expertise and readiness to handle crises have emerged as central campaign issues. Both Obama and McCain sit on U.S. Senate committees that deal with security issues.

Their running mates can also receive briefings. Fingar said he did not know whether Alaska Gov. Sarah Palin, McCain's running mate, had a security clearance yet.

INTELLIGENCE ANALYSIS

The United States is engaged in wars in Afghanistan and Iraq and pursuing a war against terrorism initiated by Bush. The new president will face a steep learning curve on the breadth of intelligence issues, and will also be expected to bring his own policy agenda, Fingar said.

This will force the agencies to freshen their analyses from across the globe. The Bush administration in its later years has usually needed updates only from fast-changing places, such as Iraq and Afghanistan. Comprehensive intelligence updates lagged from parts of Africa, Southeast Asia and Latin America, but are being completed, Fingar said.

"People will want a fresh look at the issues," he said. "We have to be ready to go on January 21," the day after the new president's inauguration, he said.

It was unclear whether a new full intelligence estimate would be produced on Iran's nuclear ambitions, one of the most controversial issues. An estimate earlier this year which concluded Iran had suspended its nuclear warhead design effort was sharply criticized by lawmakers suspicious of Iran.

Fingar said intelligence analyses were continually being updated, but said Iran's recent tests of multistage rockets were not in themselves an occasion for reopening the formal estimate, which represents an exhaustive assessment.

One of Obama's senior foreign policy aides said last week the Democrat could open talks with Iran on its nuclear program early next year if he wins. McCain has criticized Obama as too willing to talk to Tehran without preconditions, although he has supported limited Bush administration contacts.

Fingar cautioned against any new intelligence overhaul after a wrenching restructuring passed by Congress in 2004.

The reorganization followed intelligence failures before the September 11, 2001 attacks and inaccurate assessments of Iraq's weapons capabilities that led to the U.S. invasion in 2003.

Fingar said the agencies had made significant improvements and had added staff since September 11. About 55 to 60 percent of the work force has joined since 2001, he said.

Any new overhaul would risk that progress, he said. He also said there was no established precedent for leadership transition at the Office of the Director of National Intelligence, created in the 2004 overhaul to oversee the U.S. intelligence apparatus.

(Editing by Anthony Boadle)