At least 21 people have been injured after a bomb exploded on a bus in Jerusalem.

"There is no doubt that this was a terror attack," Jerusalem police commissioner Yoram Halevy said.

He added that it was too early to know who was responsible or whether it was a suicide attack.

"We are investigating where the explosive device came from, who planted it, how it got on the bus. All this is in the initial stages of investigation," he said.

Emergency workers search the scene after a blast on a bus in Jerusalem (Reuters)

Local media reported that one man who was injured was under investigation. It is thought he was not carrying any identification papers.

Images on social media showed smoke billowing from the burnt out bus on Hebron Road, in the south-west of the city.

Israeli security officials search the remains of a burnt out bus following an explosion in Jerusalem (AP)

The Magen David Adom rescue service said 21 people had been wounded, including two seriously.

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

Tensions have been brewing over a series of skirmishes between Israeli forces and Palestinian militants.

Jerusalem explosion in pictures Show all 8 1 /8 Jerusalem explosion in pictures Jerusalem explosion in pictures A bus exploded in the heart of Jerusalem wounding at least 15 people who appeared to have been in an adjacent bus that was also damaged. AP Jerusalem explosion in pictures A bus exploded in the heart of Jerusalem wounding at least 15 people. AP Jerusalem explosion in pictures Emergency workers search the scene after a blast on a bus in Jerusalem. Reuters Jerusalem explosion in pictures Israeli security officials search the remains of a burnt out bus following an explosion in Jerusalem, Israel, 18 April 2016. (EPA) EPA Jerusalem explosion in pictures Israeli police officers examine a burnt bus in Jerusalem Jerusalem explosion in pictures Israeli police check a burnt-out bus following an explosion in Jerusalem. (AFP) AFP Jerusalem explosion in pictures Israeli security officials search the remains of a burnt out bus following an explosion in Jerusalem. Jerusalem explosion in pictures An Israeli firefighter is seen inside a burnt bus in Jerusalem. (AP) AP

Around 189 Palestinians, 28 Israelis and two American citizens have been killed since the latest wave of violence broke out in October.