By Shinichi Saoshiro

TOKYO (Reuters) - Japanese stocks are expected to gain nearly 5% this year, a Reuters poll found, with monetary easing by central banks seen as a helpful factor, though the deepening U.S.-China trade war is a major source of concern.

The median from estimates by 17 analysts and fund managers polled Aug. 14-27 put the Nikkei benchmark at 21,000 at the end of 2019, a gain of 5% from last year's ending point of 20,014.

Forecasts ranged from 18,750 - a fall of 6.3% - to as high as 24,000, which would be a 19.9% gain from the end of 2018.

About 80% of the respondents expected monetary easing by central banks conducted on a global scale to be positive for the equity market.

With the global economy facing downside risks due to trade tensions, many central banks have begun lowering interest rates, collectively launching the biggest monetary easing stimulus since the financial crisis more than a decade ago.

"Any underperformance by the global economy is likely to be temporary due to the various monetary policy easing and fiscal stimulus measures," said Hideshige Watanabe, head of economic research at Sumitomo Mitsui DS Asset Management.

"Global economic expansion is seen going above its potential growth rate in the long run, lifting equities in turn."

Still, the year-end median forecast of 21,000 was lower than that of 22,375 in the previous poll three months ago. Since then, the trade war between Washington and Beijing intensified and cast a growing shadow over global markets.

On Aug. 23, China unveiled retaliatory tariffs on $75 billion of U.S. goods, and U.S. President Donald Trump immediately struck back by heaping an additional 5% duty on some $550 billion in targeted Chinese goods.

"Global monetary policy easing cannot continue indefinitely. And monetary policy can only do so much to combat uncertainty stemming from political developments," said Harumi Taguchi, principal economist for Japan and Pacific Islands at IHS Markit.

"Considering such factors, the positive impact equities derive from easy monetary policy will eventually fade the longer the spat between the United States and China continues."

Others saw potential downside risks to Japanese stocks coming from their U.S. peers, which soared to record highs in July on optimism about future Fed interest rate cuts.

U.S. stocks, which have been boosted recently by monetary easing hopes, have reached lofty levels and appear ready for adjustments, said a fund manager who could not be named due to company policy.