Japan's Nikkei stock market rose to its highest in more than 18 years Wednesday, buoyed by optimism that Greece is moving closer to a bailout deal with creditors, while China shares appeared to gain some footing following a volatile streak.

The Nikkei 225 was last up 0.5% at 20922.38, the highest since December 1996.

Elsewhere in the region, the Shanghai Composite was up 0.5%, the Hang Seng Index gained 0.3% and Australia's S&P ASX 200 was up 0.1%. U.S. stocks also rose overnight, with the Nasdaq Composite reaching an all-time high for the second straight day.

Differences remain between Greece and its lenders, but progress has been made in talks on an agreement that would stop the country defaulting and possibly leaving the eurozone.

"The global environment is OK," said Hugh Young, who helps manage Asian equities at Aberdeen Asset Management Asia Ltd.

Japan shares have had a steady run since October. Support has come from buying by the government's huge pension fund, a weaker Japanese yen and buying by foreigners. The Nikkei last broke through the 20000 level in late April.

Local investors are also more interested in equities as Japanese bonds are returning low yields, said Mr. Young.

In China, the Shanghai benchmark built on Tuesday's 2% gain. The market was recovering after a volatile session where analysts said margin calls—when brokerages ask investors to add more money to cover loans to buy stocks—caused the benchmark to plunge nearly 5% intraday.

Write to Chao Deng at Chao.Deng@wsj.com

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