After a quarter of the trading day, the Dow Jones industrial average has lost 0.4 percent, while the broad S&P 500 index and the technology-heavy Nasdaq composite index are down 0.2 percent each.
Earlier on Wednesday, Donald Trump threatened that Iran would "pay the price" for taking too long to respond in the peace negotiations.
Investors are also likely weighing recent statistics showing that US inflation rose to 4.2 percent in May, which was in line with analysts' expectations.
The inflation figure, which shows the increase in consumer prices compared to a year ago, compares with 3.8 percent in April.
