TOKYO, April 21 (Reuters) - Japanese shares extended a steep fall on Wednesday as fears of possible coronavirus-related lockdowns in Japan's biggest cities cast doubts over the prospects of an economic reopening.

Nikkei share average tumbled 2.23% to 28,451.64, while the broader Topix fell 2.24% to 1,881.11, after losing the most in about a month in the previous session.

"Global investors find little reason to buy Japanese stocks because Japan is lagging behind other countries in terms of containing the virus," said Hideyuki Ishiguro, senior strategist, Daiwa Securities.

"Worsening the sentiment is that Japan is now planning to declare a state of emergency again."

Quasi-emergency measures to curb the spread of the virus have been imposed in some parts of Japan and Osaka requested a full emergency declaration on Tuesday amid a rebound in cases. Tokyo may follow later in the week with a similar request, local media said.

Japan has vaccinated about 1% of its population, compared with 2.9% in South Korea, and at least 40% in both the United States and Britain, according to a Reuters tracker.

Steel makers and other material sectors lost the most on the Nikkei index.

Nippon Steel tumbled 5.92%, becoming the biggest loser on the Nikkei, while JFE Holdings and Kobe Steel lost 5.74% and 5.39%, respectively.

Toshiba fell 4.37% after it dismissed a $20 billion buyout offer from CVC Capital Partners.

Rakuten Group was down 5.4% following a report that the United States and Japan will jointly monitor the e-commerce firm after a unit of Tencent became a major shareholder.

SoftBank Group edged up 0.79% following a local media report the tech start-up investor would post a net profit of more than 4 trillion yen ($37.05 billion) for the year ended March. ($1 = 107.9700 yen) (Reporting by Junko Fujita; Additional reporting by Rocky Swift; Editing by Shounak Dasgupta)