The deadline for continued negotiations between Iran and the United States expired on Monday. President Trump says he is not willing to extend it.
The price of oil rose on Tuesday (Monday evening US time) and uncertainty has led to broad declines on Asian stock markets.
Japan's Nikkei 225 is down 1.6 percent at lunchtime and the broader Topix is down 0.5 percent.
Similar movements are seen in China, with Shanghai down 0.6 percent and Shenzhen down 1.4 percent. Hong Kong's Hang Seng is down 0.5 percent.
South Korea's Kospi is also falling, by 0.8 percent.
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