WASHINGTON — President Bashar al-Assad of Syria has approved the use of chlorine gas in an offensive against the country’s last major rebel stronghold, U.S. officials said, raising the prospects for another retaliatory U.S. military strike as thousands try to escape what could be a decisive battle in the seven-year-old war.

In a recent discussion about Syria, people familiar with the exchange said, President Donald Trump threatened to conduct a massive attack against Assad if he carries out a massacre in Idlib, the northwestern province that has become the last refuge for more than 3 million people and as many as 70,000 opposition fighters that the regime considers to be terrorists.