Japan's export growth slowed for a second straight month in May, with weak external demand now threatening to erode economic growth in the current quarter.

Ministry of Finance (MOF) data showed exports grew 2.4 percent year-on-year in May, less than a 3.5 percent gain seen by economists and a 8.0 percent rise in the prior month. Compared with April, exports fell 2.7 percent.

Export growth, in tandem with domestic demand, is seen as crucial for building on the momentum evident from recent data showing stronger-than-expected economic expansion - led by corporate capital expenditure at the start of the year.

The data will be scrutinized by the Bank of Japan, which is expected to keep its monetary stimulus program unchanged at its policy review later this week, after having offered a slightly more upbeat view of the economy last month.

"Exports are undoubtedly weak and the momentum is stalling due to slowdown in China," said Takeshi Minami, chief economist at Norinchukin Research Institute.

"Exports are likely to keep a drag on economic growth, which could grind to a halt or turn negative in the current quarter given weak private consumption and external demand."

EXPORT VOLUME DOWN

The Japanese currency hit a 13-year low above 125 yen to the dollar JPY= this month before gaining some ground, down about 20 percent for the year.

But weak external demand caused export volume to fall 3.8 percent in the year to May, with the volume of car shipments declining for the first time in six months.

Shipments to China, Japan's largest trading partner, grew 1.1 percent, slowing from a 2.4 percent gain in April. China-bound car exports nearly halved from a year ago.

Exports to Asia, which account for more than half of Japan's shipments, rose 3.3 percent, after a 6.0 percent gain in April.

Exports to the United States, a key market for Japanese goods, rose 7.4 percent in the year to May, slowing sharply from the prior month's 21.3 percent gain. U.S.-bound export volume fell for the first time in six months.

Imports fell 8.7 percent in the year to May to below 6 trillion yen ($48.61 billion), the lowest since December 2012, due to annual declines in energy prices.

That brought the trade balance to a deficit of 216 billion Japanese yen, roughly in line with a 226.0 billion yen deficit expected.

Revised gross domestic product data this month showed external demand knocked 0.2 percent off first-quarter economic growth as imports outpaced exports, even though the economy had shown unusually strong annualized growth.

"To be sure, output expanded at an annualized rate of 3.9 percent last quarter. But we think that growth will slow sharply

in the second quarter," Capital Economics said in a note.

"Firms' projections for industrial production, which tend to overstate the future strength of output, suggest that GDP may have stagnated in Q2. We expect growth to remain tepid in the second half of the year."