Stocks tumbled on Thursday, adding to a string of recent losses, even after the Trump administration offered a reprieve on tariffs on Canada and Mexico.

The S&P 500 dropped 1.8 percent, taking the slide for the index this week to 3.6 percent and putting it on course for its worst week since a banking crisis two years ago that felled some of the country’s small lenders.

Mr. Trump had imposed tariffs of 25 percent on imports from Mexico and Canada on Tuesday. He also raised tariffs on imports from China. Investors are concerned that the tariffs — and retaliation from the other countries — will fuel inflation, slow economic growth and hurt American businesses.

After sliding 3 percent over Monday and Tuesday, the S&P 500 staged a small recovery on Wednesday as the big tech stocks that have driven the market in recent years bounced and investors grew hopeful for some reprieve on tariffs. But the administration’s move to exempt auto manufacturing, along with products already part of a previous trade deal between the United States, Mexico and Canada, until April failed to settle investors’ concerns about the impact on the economy.

The S&P 500 is now almost 7 percent below a record set just over three weeks ago, which means it is close to falling into correction, defined as a sell-off of 10 percent or more from a recent peak. The tech-heavy Nasdaq Composite index and the Russell 2000, which tracks smaller companies that are typically more exposed to the economy, are both already in correction.