The UN human rights office has said that an 18-month jail sentence handed down to an Israeli soldier for killing a wounded Palestinian man was "excessively lenient" and "unacceptable".

Elor Azaria, the soldier who shot Abdel Fattah al-Sharif Elor in the head after he was incapacitated following a knife attack in March 2016, was sentenced to 18 months' imprisonment on Tuesday.

"We are deeply disturbed at the lenient sentence given by the Tel Aviv Military Court earlier this week to an Israeli soldier convicted of unlawfully killing a wounded Palestinian in an apparent extrajudicial execution of an unarmed man who clearly posed no imminent threat," Ravina Shamdasani, UN human rights spokesperson, said in Geneva on Friday.

READ MORE: Elor Azaria verdict - 'No justice for Palestinians'

Manslaughter carries a maximum punishment of 20 years under Israeli law, she said.

"This case risks undermining confidence in the justice system and reinforcing the culture of impunity," she said.

"This is a chronic culture of impunity we are talking about."

More than 200 Palestinians have been killed by Israeli security forces since the most recent surge in violence began in the West Bank in September 2015, she said.

Azaria is the only member of the Israeli security forces to have been brought to trial for such a killing, Shamdasani said.

READ MORE: Israel says no more visas for 'biased' HRW staff

The 18-month jail sentence has also prompted Palestinian criticism.

Palestinian President Mahmoud Abbas is due to address the UN Human Rights Council in Geneva on Monday at the start of its main annual session that lasts until March 24.

Israel, backed by the US, says that the 47-strong forum is biased against it due to its frequent resolutions condemning illegal Israeli settlements in the occupied West Bank and practices in the Gaza Strip.