South Africa's President Cyril Ramaphosa has hit out at the main opposition party after one of its election adverts featured a national flag engulfed in fire.

The Democratic Alliance (DA) has defended the image saying it wanted to highlight its argument that the country is in trouble.

Burning the flag is not illegal, but the president called the DA's depiction an "abuse", adding that it undermined the unity of the country.

Campaigning has become increasingly fractious ahead of the 29 May general election.

In the advert, as the flag is seen going up in flames, a voice-over warns "life will only get worse" under the African National Congress (ANC), which has been in power since 1994.

The burning of the flag is then reversed as the speaker urges voters to support the DA.

"This election is about survival," it ends as the flag is replenished and returns to its original form.

The DA advert angered some South Africans who accused the party of desecrating the flag to highlight challenges faced by citizens.

In a statement on Tuesday, the president said while he was committed to free speech, this has to "be exercised with respect for all people and that it does not incite social upheaval or violence".

In his campaigning, Mr Ramaphosa has acknowledged that there are problems in the country, but he has also emphasised the transformation that South Africa has made in the last three decades.

The ANC led the struggle against white-minority rule and the legalised form of racial discrimination known as apartheid.

"South Africans are more educated, empowered and healthier than they were under apartheid," Mr Ramaphosa has said, urging people not to threaten that progress.

Addressing the media on Sunday, when the advert was launched, DA leader John Steenhuisen said it was a "symbolic representation of the future that awaits South Africa if people do not vote for the DA".

“This is our fourth advert, and nobody has commented on the others, which surprises me," DA official Ashor Sarupen told the Daily Maverick news website.