A knife-wielding woman stabbed three people in Brussels on Monday and was later shot by police when she refused to obey their orders, prosecutors said.

Police cars and officers converged on the area near a bus stop where the attack happened in the Uccle neighborhood of the Belgian capital, while a police helicopter flew overhead.

The attacker stabbed and lightly wounded two people on a bus, then fled and stabbed a third person at a nearby shopping center. The reason for the attack remains unclear.

The three victims "were lightly injured and they were all taken to hospital for first (aid) care," said Ine Van Wymersch, spokeswoman for the Brussels prosecutor's office.

"The woman didn't listen to police so they were forced to use their (fire) arm," Van Wymersch said.

Belgium has been on alert since suicide bombings on Brussels' airport and subway killed 32 people on March 22. The attack was claimed by the Islamic State group.

Several crime incidents and hoax calls in recent weeks have seen police deploy in force.

But Van Wymersch said authorities didn't have "any indication" that the stabbing Monday was related to terrorism.

Belgian media said the woman was apparently aged in her 50s and appeared to be mentally unstable.