A fire at the office of an Israeli NGO that champions human rights in the occupied Palestinian territories led to suspicions of arson on Monday, but was later attributed to a suspected electrical fault.

The blaze caused extensive damage to the Jerusalem office of the group B’Tselem. There were no injuries as the property was empty at the time, a spokeswoman for the NGO said.

Initial media reports suggested it could have been an arson attack at a time of heightened tensions between rights activists and rightwing Israeli groups, but the fire department later told B’Tselem it was a suspected electrical fire.

“The fire brigade made an official announcement to us and as far as we are concerned that is what we needed to hear,”, the spokeswoman, Sarit Michaeli, said. “We are relieved and happy that this is what it was and are going to start cleaning up ASAP.”

She said the fire would not stop the organisation’s work of documenting and exposing human rights abuses.

The building was sealed off on Monday morning and forensic teams were seen carrying bags of evidence out of the offices. Windows were smashed along the right side of the building where B’Tselem’s offices are located, while other areas appeared to have sustained little damage. The NGO said it had not yet been given access to the building nor knew the extent of the damage to its archives.

The blaze took place at a time of increased tensions. Scores of people have been killed in several months of clashes in Israel and the occupied West Bank. The wave of violence, in its fourth month, has raised fears of wider escalation, a decade after the last Palestinian uprising subsided.

Since 1 October, Israeli forces or armed civilians have killed at least 139 Palestinians – 89 of those were described as assailants by authorities; most of the others were killed in clashes with security forces.

Palestinian stabbings, car-rammings and shooting attacks have killed 21 Israelis and a US citizen. Vandalism attacks, including torchings, by suspected far-right Israeli groups have caused damage to Palestinian property and mosques and churches.

Two Israelis were charged last week over the death of a Palestinian baby and his parents in the West Bank last year after their home was set on fire.

Israel’s rightwing government has proposed legislation to limit foreign donations from governments and private benefactors to B’Tselem and many other Israeli NGOs, something that could severely restrict the organisation’s ability to operate.