The Britain-based monitoring group reported on Tuesday that US warplanes targeted military gatherings in the Al-Quriyah desert of Deir ez-Zor province. It said the strikes came after a rocket believed to be fired by "pro-Iran militiamen" landed near a US base in the countryside of al-Hasakah province in northeastern Syria, reports Xinhua news agency.

Meanwhile, intermittent explosions of unknown origin were heard at a US military base inside the al-Omar oil field base in Deir ez-Zor, the observatory said.

US fighter jets were seen flying over several villages, reaching as far as the town of Mayadeen in Deir ez-Zor's countryside near the Iraqi border, according to the observatory.

There were no immediate reports of additional casualties.

The region has seen increased tensions between US forces and Iran-backed groups, which have a significant presence in eastern Syria.