Zuma was speaking at the funeral of ANC councillor and chief whip of the Hibiscus Coast Municipality, Wandile Mkhize.

Mkhize was shot dead as he arrived home from the ANC policy conference in Midrand last week.

"As the ANC, we are always concerned when we lose members, regardless of their position in the organisation, but we are even more concerned when these deaths are targeted at ANC leaders," Zuma said.

He said the fact that Mkhize was the third ANC leader to be killed in a similar style - shot dead as they arrived home - in KwaZulu-Natal, was of grave concern.

ANC eThekwini region secretary, Sbu Sibiya, and ANC councillor Wiseman Mshibe were killed in the same way last year.

"It's becoming common in KwaZulu-Natal and it has to be investigated. KwaZulu-Natal comes from a history of violence and we thought that had passed.

"The big question we must ask ourselves is who benefits from these killings. I appeal to all ANC members to be vigilant as it seems there are dark, unforeseen forces hellbent on causing disunity and destabilising the organisation.

"Such attacks must be thoroughly investigated and perpetrators brought to book," Zuma said.

Zuma described Mkhize as a dedicated, vigilant and disciplined member of the organisation.

He said he last spoke with Mkhize at the ANC's "successful policy conference" about the term "second transition".

Zuma said Mkhize and two other members wanted his explanation of the term.

"They were not arguing the content of our policy document, but about one issue and the phrase.

"After I explained to them what we mean by second transition, he commented that he did not see why comrades were debating the term and not the unity of the organisation, which was paramount to him."

Mkhize also raised his concerns to Zuma about how the ANC lobbies leadership.

The president said Mkhize was the kind of leader who was disciplined and ready to accept the decisions of the organisation, no matter how hurtful.

"When you engaged him, you didn't have to read something out of what he said because you knew that he had no ulterior motive. His untimely death has left a gaping wound in the ANC," Zuma said.

KwaZulu-Natal Premier Zweli Mkhize warned that, while they did not know the motive for the killing, the ANC should watch the influence of business on politics.

"This is one area that will destroy the organisation. It's happening at national [level] and we have seen it at eThekwini as well."

Young Communist League national secretary Buti Manamela said Mkhize was radical, militant and remained respectful to senior leaders.

"Even when his region was disbanded, he never challenged the decisions of the leadership. He believed that, no matter how painful the decision was, it had to be respected. But his killing was political," Manamela said.