WASHINGTON—Britain, France and the U.S. united Thursday around broad plans for a military strike against Syria as they worked to bridge differences over the scope and purpose of a coordinated response to a suspected chemical weapons attack, U.S. officials said.

President Donald Trump met with his national security team on Thursday to weigh military options while Syrian President Bashar al-Assad sought to limit the impact of an expected attack by moving warplanes under the protection of Russian air defenses.

While...