The African National Congress (ANC) said late on Thursday that several other parties had agreed to join it in a unity government but the details were not yet finalised, the night before the newly-elected parliament was due to sit.

The former liberation movement has been holding hurried talks with rivals to form a governing alliance after it lost its parliamentary majority in a May 29 election.

The last-minute talks underscored the uncertainty in Africa's most advanced economy, where only the ANC has governed since the end of apartheid and now parties with sharply opposing views must find a way to share power.

The broad parameters of a deal need to be thrashed out by Friday, when lawmakers will elect the speaker of parliament and the country's president. President Cyril Ramaphosa is likely to be re-elected as the ANC still has the most seats, but other key posts in government are seen as up for grabs.

"We are talking to political parties as we speak right now," ANC Secretary-General Fikile Mbalula told reporters after a meeting of the party's top officials, adding he was not yet in a position to give details on those talks.

He said parties including the pro-business Democratic Alliance (DA) had agreed on forming a government of national unity, which will likely reassure investors. But he made clear the ANC and DA had yet to nail down exactly how they would cooperate.

"If the DA were to get some of these things that it wants it means the ANC will be dead," Mbalula said.

The DA's top leaders were also holding closed-door talks on Thursday evening. The party has not yet confirmed it will join the government.

Many ANC supporters strongly oppose a partnership with the DA because it is seen as a defender of the interests of the privileged white minority, although it denies this.

The ANC won 159 out of 400 seats in the National Assembly. The DA is its nearest rival, with 87 seats, while the populist uMkhonto we Sizwe (MK) party led by former President Jacob Zuma has 58.

The EFF, led by former ANC youth leader Julius Malema, came fourth, winning 39 seats, and the Inkatha Freedom Party (IFP) fifth with 17.

The IFP has already announced it will join the unity government, while the EFF has said it will not.