TOKYO, March 5 (Reuters) - Japan's Nikkei share average extended losses into a fourth day as concerns over a global trade war dented sentiment, with steelmakers, automakers and shipping companies underperforming the market.

The Nikkei fell 0.5 percent to 21,075.44 in midmorning trade, after the yen got a lift from safe-haven flows as risk sentiment soured after U.S. President Donald Trump proposed tariffs on imported steel and aluminium.

Steel shares tumbled again, with Nippon Steel & Sumitomo Metal dropping 1.5 percent, JFE Holdings and Kobe Steel sliding 1.5 percent.

Automakers, big consumers of steel and aluminium, also lost ground. Honda Motor Co fell 1.8 percent, Toyota Motor Corp dropped 1.4 percent and Subaru Corp shed 1.1 percent.

Shippers were sold, with Mitsui OSK Lines falling 2.5 percent and Kawasaki Kisen off 2.1 percent.

With the dollar falling for a fourth straight session, investors have started to worry that Japanese companies will have to lower their dollar-yen assumptions for the next fiscal year, which would impact their profit outlook.

The dollar stood at 105.43 yen, slightly above Friday's low of 105.23, a level not seen since November 2016.

The broader Topix dropped 0.7 percent to 1,696.91.

(Reporting by Ayai Tomisawa; Editing by Sam Holmes)