WASHINGTON — American warplanes in Syria attacked a pro-government convoy on Thursday after it ignored warnings and violated a restricted zone around a base where United States and British Special Forces train Syrian rebels to fight the Islamic State, Pentagon officials said.

The American strike, corroborated by both pro- and anti-government fighters on the ground, was at least the third time in recent months that the United States military has attacked forces loyal to President Bashar al-Assad of Syria.

On Thursday, American officials said, more than 20 vehicles drove within 18 miles of the al-Tanf base in southern Syria housing the American and British Special Forces, breaching the so-called deconfliction zone radius around the encampment.

Low-flying American aircraft buzzed the convoy and fired a warning strike, trying to get the vehicles to reverse course, the Pentagon officials said, but when the convoy continued, the planes opened fire.