JERUSALEM - An explosion aboard a bus in Jerusalem caused around 20 casualties on Monday, Israeli media said, and police said it may have been a militant attack or the result of a fire from other causes.

Television images showed smoke billowing from two burned-out buses, one of which apparently set fire to the other, on Derech Hebron, an area in southwest Jerusalem close to the boundary with the Israeli-occupied West Bank. Smoke billowed over the horizon.

It was not immediately clear if the casualties included fatalities. Israel's Magen David Adom ambulances service said it had taken 10 wounded people to hospitals and that others remained at the site of the blast.

Attacks on Israeli buses by suicide bombers were a hallmark of the Palestinian uprising in 2000-2005 but have been rare since.

An Israeli police spokeswoman initially said there were indications that Monday's incident was a militant attack, but in an updated statement said it may have begun when an empty bus caught fire, ignited another bus that had passengers aboard.