A school bus arriving outside a Seattle elementary school to pick up children at the end of the day on Thursday was struck by gunfire, prompting a lockdown of the school and a search for suspects, but no one was hurt, city police and school officials said.

The bus driver heard loud noises, then saw that two windows in the vehicle had been shattered and realized that the bus was under fire before he took shelter inside the school, Seattle Public Schools spokeswoman Stacy Howard said.

No children were on the bus at the time, she said.

The shooting occurred shortly before 4 p.m. at the Martin Luther King Jr. Elementary School, which has a student body of about 370 children in kindergarten through fifth grade, Howard said.

The school was placed on lockdown, with all staff and students instructed to stay inside, as police arrived on the scene to secure the area and search for suspects, she said.

As a precaution, students and staff at another elementary school and a middle school in the vicinity were also ordered to "shelter in place," she said.

A Twitter message posted by Seattle Public Schools said: "School bus hit by gunfire. Damaged, but no injuries. Driver was on bus, no students."

Howard said police were helping students leave the school after the incident through a safe exit in order to meet other school buses or parents arriving to pick them up.