The Ethiopian government has said it will launch an independent investigation into the killing of protesters by security forces during anti-government demonstrations in recent weeks.

Speaking to Al Jazeera, government spokesperson Getachew Reda insisted, however, that police violence in the country was "not systemic".

"There are cases of off-grid police officers who sometimes take the law into their own hands," he said. "The government takes such allegations very seriously."

INSIDE STORY: What is triggering Ethiopia's unrest?

According to Human Rights Watch, Ethiopian security forces have killed at least 500 people since anti-government protests began in November. Thousands of people have also been arrested and detained, the US-based rights group says.

Earlier this month, security forces killed nearly 100 people across Ethiopia in three days of violent protests, according to Amnesty International. Security forces opened fire on protesters, activists said.

Anti-government protests that started among the Oromo, Ethiopia's largest ethnic group, have spread in recent weeks to the second most populous group in the country, the Amhara.

Both groups are demanding more political and economic rights.

"The ruling party won 100 percent of federal and regional parliamentary seats in last year's election," Al Jazeera's Charles Stratford, reporting from the capital Addis Ababa, said.

"The opposition protesters say the party is dominated by a minority ethnic group that has been in power for more than 25 years and is ignoring their constitutional rights."

The Ethiopian government blamed the opposition in and outside the country for organising what it called "unauthorised protests by anti-peace forces".

'We welcome the decision'

Ethiopia has previously dismissed a plea from the UN to allow international observers to investigate the killing of protesters by security forces.

Ravina Shamdasani, a spokesperson for the Office of the UN High Commissioner for Human Rights (OHCHR), said he welcomed the Ethiopian government's decision to launch an independent probe.

"We urge the government to ensure that the investigation has a mandate to cover allegations of human rights violations since the unrest in Oromia began in November 2015," Shamdasani told reporters in Geneva on Friday.

She went on to stress that the probe should be "indeed independent, transparent, thorough and effective, with a view to establishing whether the use of excessive force occurred and with a view to bringing to justice the perpetrators of any human rights violations".

"The government has repeatedly said that security forces who have committed abuses or bear any responsibility for the killing of innocent protesters will be punished," said Al Jazeera's Stratford.

"But so far, publicly at least, no one has been called to account."