* Nasdaq hits record-high of 5,143.31
* Greece's aid could be extended till year-end - report
* Weekly jobless claims fall to 15-year low
* Fitbit shares soar in IPO debut
* Indexes up: Dow 1.05 pct, S&P 0.91 pct, Nasdaq 1.26 pct (Adds details, changes comment, updates prices)
By Tanya Agrawal
June 18 (Reuters) - The Nasdaq Composite index surpassed the 15-year all-time high it set during the peak of the dotcom bubble as more data on Thursday showed the U.S. economy was gathering steam.
The index hit 5,143.31, topping the previous high of 5,132.52 it touched on March 10, 2000. The S&P and Dow were at their highest levels in about a month.
Brian Fenske, head of sales trading at ITG in New York, said the Nasdaq was on firmer footing and there were no similarities between its rally in 2000 and the surge in recent months.
"The current rally is more durable. It is incredibly tough to go public in this market. We just didn't have this kind of scrutiny back in 2000," said Fenske.
U.S. stocks briefly extended their rally on an unconfirmed report in German daily, Die Zeit, that Greece's aid will be extended until year-end but the IMF would not take part in the financing for the "time being".
Greek Finance Minister Yanis Varoufakis presented new proposals to counterparts in the euro zone meeting in Luxembourg, a Greek government official said.
The Fed said on Wednesday that the U.S. economy was likely strong enough to withstand an interest rate increase later this year but cut its economic growth forecasts for 2015.
Even though a majority of Fed officials continue to see higher rates by the end of 2015, they expect rates to rise slightly less by the end of 2016 and 2017 than they did in their March forecasts.
U.S. consumer prices in May recorded their largest increase in more than two years as gasoline prices surged, while factory activity in the U.S. mid-Atlantic region expanded in June at a faster pace than expected.
Other data showed the labor market tightened as first-time applications for unemployment benefits declined to a near 15-year low last week.
At 13:31 p.m. ET (1731 GMT) the Dow Jones industrial average was up 188.94 points, or 1.05 percent, at 18,124.68, the S&P 500 was up 19.16 points, or 0.91 percent, at 2,119.6 and the Nasdaq Composite was up 63.62 points, or 1.26 percent, at 5,128.50.
All the 10 major S&P 500 sectors were higher with the health index leading with a 1.4 percent rise.
Microsoft's 1.4 percent rise lifted the Nasdaq and the S&P 500, while 3M's 1.9 percent gain was the biggest boost to the Dow. All 30 Dow components were in the black.
Fitbit shares ran up as much as 60 percent to $31.90 in their debut, valuing the maker of popular wearable fitness-tracking devices at $6.5 billion.
Oracle shares fell as much as 8.7 percent to $40.97 - a nearly six-month low - a day after the company forecast a quarterly profit below analysts' estimates.
Advancing issues outnumbered decliners on the NYSE by 2,228 to 775. On the Nasdaq, 1,978 issues rose and 761 fell.
The S&P 500 index showed 40 new 52-week highs and two new lows, while the Nasdaq recorded 144 new highs and 25 new lows.
Reporting by Tanya Agrawal; Editing by Don Sebastian