Oil prices resumed their downward slide after the OPEC Plus cartel of oil producers said over the weekend that it would pump more oil, despite concerns that President Trump’s trade war will curb demand.

The U.S. benchmark oil price fell to around $56 a barrel, from $58 on Friday. For many companies, the steady decline means it will not be profitable to drill wells in the United States despite Mr. Trump’s calls for increased production.

Prices were last around this level in early April, just before Mr. Trump said he would pause reciprocal tariffs on most countries for 90 days. That announcement led to rallies in both the stock market and the oil market, though oil prices have since waned.