* All sectors in positive territory

* Market takes comfort from Yellen's 'patient' comment

* Immediate resistance seen at 25-day moving average

By Ayai Tomisawa

TOKYO, Dec 18 (Reuters) - Japan's Nikkei share average jumped 2.5 percent on Thursday morning after the U.S. Federal Reserve gave an upbeat assessment of the U.S. economy and promised to take a patient approach towards raising interest rates.

The Nikkei was at 17,242.15 in mid-morning trade, posting its biggest one-day gain since early November and pulling away from a 6-1/2-week low of 16,672.94 hit on Wednesday.

An immediate resistance level is seen lurking at its 25-day moving average of 17,404.44, analysts said.

Following a two-day meeting, the U.S. central bank gave a strong signal that it was on track to raise interest rates sometime next year.

The Fed said it would take a "patient" approach in deciding when to bump borrowing costs higher, which Fed Chair Janet Yellen, at a news conference, defined as "at least a couple of meetings."

"Investors took heart from her comment that the Fed will take a patient approach about raising interest rates, and Yellen was clearly making an effort to communicate well with the market," said Hiroyuki Nakai, chief strategist at Tokai Tokyo Research Center.

Also soothing market sentiment was a rebound in world oil prices, which eased concerns about the oil-dependent Russian economy. U.S. crude futures ended higher for a second day.

Nakai said falling oil prices were hurting Russia particularly hard, but he said long-term weakness in crude prices would be positive for most advanced economies such as the United States and Japan which are importers of oil.

"The rally in oil prices is expected to be temporary. I expect oil prices to stay low, and oil importers like Japan will benefit from low oil prices in the long term," he said. "The market will likely gradually start pricing in such a positive factor."

Exporters gained after the dollar climbed to 118.85 yen from a low of 116.29 on Wednesday.

Toyota Motor Corp jumped 3.4 percent, Honda Motor Co gained 2.1 percent and Sony Corp climbed 4.1 percent.

Tokyo Electric Power Co surged 4 percent after saying it would not raise electricity rates in 2015 and forecast another year of profits for the year to next March.

The broader Topix index gained 2.5 percent to 1,385.49, with all of its 33 subsectors in positive territory. The new JPX-Nikkei Index 400 also added 2.5 percent to 12,561.25.

(Editing by Edmund Klamann)