TOKYO (Reuters) - Japan's exports rose in April from a year earlier led by US-bound shipments of cars, but the monthly pace of growth slowed from March - a worrying portent for external demand as a slowdown in the economies of China and the U.S. clouds the outlook.

Ministry of Finance data out on Monday showed exports grew 8.0 per cent year-on-year in April, versus a 6.4 per cent gain seen by economists in a Reuters poll, slowing from a 8.5 per cent gain in March. Compared with March, shipments fell 1.5 per cent in April.

Slowdown in export growth could hold back the economy's recovery from last year's recession, after last week's data that showed the economy posted two straight quarters of moderate expansion.

Policymakers count on exports as a key driver of corporate activity, which could help boost wages and spur private spending to generate a virtuous growth cycle needed to shake off years of deflation.

"The figures were not so bad as we thought but shipments to China and Asia turned out weak, which is worrying for the outlook for Japan's external demand," said Hidenobu Tokuda, senior economist at Mizuho Research Institute.

Shipments to China, Japan's largest trading partner, rose 2.4 per cent, slowing from a 3.9 per cent gain in March, the data showed. China-bound car exports halved in the year to April.

Exports to Asia, which account for more than half of Japan's shipments, grew 6.0 per cent, slowing from a 6.7 per cent gain in March. Tokuda said the slowdown probably reflected weakness in Thailand and other ASEAN economies.

Exports to the United States rose 21.4 per cent in the year to April, keeping the pace of gains in the previous month with brisk shipments of cars and vehicle engines.

Imports fell 4.2 per cent in the year to April, a fourth straight month of annual declines, mainly due to a sharp fall in crude oil prices. The fall was deeper than the median estimate for a 1.5 per cent drop.

The trade balance swung back into a deficit of 53.4 billion yen (S$590 million), much smaller than the median estimate for a 318.9 billion yen deficit, after posting the first trade surplus in nearly three years in March.

First quarter gross domestic product data showed export growth slowed to 2.4 per cent from 3.2 per cent in October-December, reflecting the slowdown in the U.S. and Chinese economies.

Imports outpaced exports, causing external demand to shave off 0.2 per cent off the economic growth in January-March.