About 15 minutes into trading, the Dow Jones Industrial Average stood at 24,016.61, down 0.6 percent.

The broad-based S&P 500 dropped 0.4 percent to 2,638.86, while the tech-rich Nasdaq Composite Index fell 0.4 percent to 7,037.91.

The losses extended the downward trend from Monday's session, when US stocks opened higher but slid into negative territory on worries about US policies on Iran and trade.

The US late Tuesday delayed by 30 days imposition of tariffs on steel and aluminum imports from Canada, Mexico and the European Union while they continue to negotiate a compromise to reduce import levels. 

The EU sharply criticized the US for prolonging "market uncertainty" and called on the administration of President Donald Trump to cancel the levies.

Analysts are also looking ahead to the Fed's policy announcement Wednesday following a two-day meeting. While no rate hikes are expected this week, the communique will be scrutinized for signs on whether the US central bank plans to speed up its rate hikes in response to signs of higher inflation.

Apple shares gained 0.2 percent ahead of its earnings, which will be released after the stock market closes. Some analysts expect disappointing iPhone sales to be offset by new announcements on shareholder buybacks or dividends.

Pfizer dropped 1.9 percent, despite reporting a 14.1 percent rise in first-quarter earnings to $3.6 billion, as revenues lagged analyst expectations.

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