Zimbabwe's political rivals have clashed over an apparently secret document said to be from intelligence sources that alleges an outlandish plot by President Robert Mugabe to hire a Lebanese murder squad to assassinate his South African counterpart, Jacob Zuma, and a top diplomat.

The typed two-page report – persuasively detailed but at times written in excitable language strewn with grammatical and spelling errors – has been dismissed by Mugabe's Zanu-PF party as "hogwash" and a "typical setup". The rival Movement for Democratic Change said the assassination threats should be taken seriously.

They come amid souring relations between neighbours Zimbabwe and South Africa. Last week Mugabe branded Zuma's chief mediator, Lindiwe Zulu, "some stupid, idiotic woman" and a "little street woman" after her failed attempt to force a postponement of elections.

The purportedly leaked document, dated 2 July and of unknown authorship, contains a paragraph headed: "Lebonese assasins" (sic). It states: "On Monday this week Mugabe hired six Lebanese nationals to try and assassinate Lindiwe Zulu, who is Zuma's advisor. The six met clandestinely with Mugabe yesterday to be briefed by him on the details of their mission … The six were told they must not concentrate on Zulu only, but should also pay attention to Zuma himself, and if they get a chance to do so they must assassinate him as well – but everything must appear as an accident."

It adds: "Mugabe promised the six Lebonese [sic] an undisclosed fortune in cash if they succeed in getting rid of the two who [sic] South Africa senior officials who are giving him a lot of trouble. Names of the six could not be established, but they entered into the country via Zambia in the last few days, where they had previously held a close meeting with [president] Michael Sata before travelling into Zimbabwe."

The document also claims that Mugabe summoned and castigated security officers over a leak to the MDC leader, Morgan Tsvangirai, regarding the involvement of an Israeli company in managing the country's electoral role. Mugabe ordered that "the culprits who leaked the information are made to suffer and die a painful death", it says. Tsvangirai has spoken publicly about his concern over the role of the Israeli company Nikuv.

The Guardian has seen a copy of the two-page document but has not been able to verify it as being genuine. The possibility that it is a hoax designed to embarrass the opposition cannot be ruled out.

Rugare Gumbo, spokesman for Zanu-PF, insisted: "To be frank with you, it's all rubbish and hogwash to think a head of state like president Mugabe would set up something like that. We have a disagreement with Lindiwe Zulu but it would not go to that extent."

Asked about the alleged threat to Zuma, he replied: "You should not take it seriously. It's a typical setup in Zimbabwe to try and cause some confusion before the election because we are going ahead with our election according to our laws whether they like it or not."

But this is not the first time allegations have surfaced of Zimbabwean government agents carrying out clandestine operations in South Africa. Nor are rumours of assassinations so unusual: South Africa's Mail & Guardian newspaper recently compiled a list of nine Zimbabwean politicians who have died in mysterious car crashes, including the one last month involving Edward Chindori-Chininga, the outspoken chairperson of a parliamentary committee that had just released a highly critical report on the diamond industry.

Fierce factional infighting within Zanu-PF and the security sector has resulted in a number of damaging intelligence leaks, notably on the Facebook page of a mole known as Baba Jukwa.

Douglas Mwonzora, the MDC's spokesperson, said of the document: "We have had sight of the intelligence information but we are unable to confirm its authenticity. For the MDC, the international community must take these assassination allegations seriously."

But Roy Bennett, the MDC treasurer-general, in exile, was more certain. "We know the report to be credible; it comes from reliable and trustworthy sources within the heart of the CIO [Central Intelligence Organisation]," he said. "It is a stark reminder of the full horrific extent that Robert Mugabe is prepared to go in order to hang on to power and avenge his critics."

The MDC and civil society groups in Zimbabwe have applauded Zuma for taking a tougher stand against Mugabe than the previous South African president, Thabo Mbeki.