A bomb blast ripped through a bus in Jerusalem on Monday and sparked a fire, wounding at least 21 people, Israeli police said, in an apparent escalation in a wave of violence.

Details were still emerging, but police said a bomb had exploded on one bus in a relatively isolated area of Jerusalem, sparking a fire that spread to another one as well as a car.

Israeli domestic security agency Shin Bet referred to the explosion as a "terror attack".

The bombing was expected to lead to a sharp increase in security ahead of Jewish Passover celebrations beginning Friday night.

If confirmed as a Palestinian bombing, it would both reverse a decline in a wave of violence that erupted in October and mark an escalation, with most of the attacks having been stabbings.  

"A professional examination of police sappers has proven that a bomb exploded on the back part of the bus, resulting in the wounding of passengers and the burning of the bus," a police statement.

"In addition, another bus and car were damaged."

An AFP journalist at the scene said one bus was completely burnt out while another was partially burned, with a large contingent of firefighters battling to extinguish the blaze.

Police said 21 people were injured, with medics reporting at least two hurt seriously. Police were investigating whether any of the wounded were behind the bombing.

Authorities initially said most of the wounded were passengers on the second bus, though conflicting information later emerged.

Security implications  

The blast struck in an area of the city without any major buildings or homes and which is not heavily used by pedestrians.

The location was on Moshe Baram Street close to the so-called Green Line dividing mainly Jewish west Jerusalem from predominantly Palestinian east Jerusalem.

Jerusalem Mayor Nir Barkat called on residents to be vigilant, "but continue with your plans".

"Here in Jerusalem and in Israel, we go back to normal life as fast as possible," he said.

"It's part of the deep understanding that if it's a terror attack, they want to deter us from our normal life, and what we must do... is go back to normal life as fast as possible."

The explosion comes with tensions high following a wave of violence that began in October that has killed 201 Palestinians and 28 Israelis.

Most of the Palestinians killed were carrying out knife, gun or car-ramming attacks, according to Israeli authorities.

The last bomb targeting a bus in Jerusalem dates back to 2011, when a British tourist was killed.

In Tel Aviv, a bomb exploded on an empty bus in 2013 in what Israeli authorities called a "terrorist" attack.

Suicide bombings were frequent during the second Palestinian intifada between 2000-2005.

Speaking before the bomb was confirmed, police spokesman Micky Rosenfeld said "if it was a terrorist attack, the implications are very great in terms of security on the ground".

Attacks have steadily declined in recent weeks, though there have been concerns the Passover holiday beginning on Friday could lead to a new surge in violence.

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