TOKYO (Dow Jones)--The Japanese economy grew an annualized 6.0% in the July-September period, posting its first gain since the economy was brought low by the March 11 earthquake and tsunami.

But the strong rise may not be an accurate reflection of Japan's true economic prowess, as much of the growth was due to pent-up overseas demand being satiated with the restoration of supply chains in sectors such as auto manufacturing that saw severe disruptions and widespread production stoppages.

Easily outshining the U.S.'s annualized third-quarter growth of 2.5%, the GDP figures released by the Cabinet Office Monday were a tad bit better than economist forecasts that the nation's real, or price-adjusted, gross domestic product would increase an annualized 5.9% from the previous quarter. GDP grew 1.5% on quarter in July-September, according to the data.

While economists expect Japan's growth to continue in the coming months, they see the third-quarter surge as unsustainable. Months of growth driven by intense national efforts to rebuild quake-damaged supply chains have moderated, while a slowing global economy and the yen's near record-high levels threaten exports.

Japan's Economy Minister Motohisa Furukawa warned that the nation's economic recovery would slow as the pace of exports moderates due to weakening overseas demand.

"We have to pay adequate attention to downside risks, such as possible deterioration in foreign economies, rapid appreciation in the yen and the impact of damage from the Thai floods," Furukawa said at a press conference after the release of the data.

Driving the third quarter growth was a 6.2% on-quarter spike in exports, a key engine of the Japanese economy. The country's auto sector was effectively brought to a standstill after the March disaster, with the July-September representing the first real resumption in shipments.

Private consumption, which was the second largest contributor to the growth in the third quarter, grew 1.0%.

Analysts noted the headline figure's gain was not surprising and to a certain extent masks what Japan's economy is truly facing.

"This reading alone warrants no optimism" as the growth is created by a rapid recovery from the March earthquake and tsunami, said Dai-ichi Life Research Institute chief economist Hideo Kumano.

"The effects of Thai floods will likely to emerge in early October, while exports to Europe will likely see a slowdown from October onward," he added, noting that "changes in external demand will likely happen in the October-December period."

Japan's GDP outcome was "in line with our expectations," said Norio Miyagawa, a senior economist at Mizuho Research and Consulting. "The large gain was expected as the economy has been making a fast-paced recovery after bottoming in April," he added.

A Japanese government official briefing reporters on the data said that it was the strongest period of growth since a 10.2% annualized expansion in the January-March quarter of 2010 and the first rise in four quarters.