South Africa's coalition government is on shaky ground, with the sharp divisions between its two biggest parties - the African National Congress (ANC) and Democratic Alliance (DA) - exposed in a crucial vote on the national budget.

The centre-right DA voted against the fiscal framework - a key part of the budget - after rejecting an increase in VAT, and demanding a cut in spending across all government departments.

The ANC, which positions itself as a centre-left party, refused to bow to what it called the DA's demand for an "austerity budget".

It demonstrated its political acumen by winning the support of a slew of smaller parties - both inside and outside government - to get the fiscal framework through parliament by 194 votes to 182.

The DA filed papers in court to challenge the vote, saying it was "procedurally flawed" while its top leadership is due to meet later to decide whether or not to remain in what South Africans call a government of national unity (GNU).

Professor William Gumede, an academic at Wits University's school of governance in Johannesburg, told the BBC it was unclear whether the DA would quit the government at this stage.

"It will be asking itself whether this is the tipping-point or whether it should wait - at least until the outcome of the court case," Prof Gumede said.

The coalition government was formed less than a year ago after the ANC lost its parliamentary majority in elections for the first time since Nelson Mandela led it to power in 1994 at the end of white-minority rule.

South Africa's business sector lobbied the two parties to enter into a coalition, seeing it as the best option to guarantee economic stability.

But hinting that the DA's participation was no longer certain, DA spokesman Willie Aucamp accused the ANC of a "serious infraction" and said the party had "crossed a line in the sand".

DA federal chair Helen Zille said the party would consider all its options, and not rush into a decision.

"We know that being in a coalition requires compromise. You can't get it all. But the ANC also can't get it all, and they are refusing, point blank, to share power," Zille added.

The ANC took an equally tough stand, with its parliamentary chief whip, Mdumiseni Ntuli, accusing the DA of "complete betrayal" by breaking ranks with its partners in the GNU.

"The DA is a member, or was a member," Ntuli said.

"I don't know what is going to happen with them now, but the GNU remains," he added, referring to the fact that other parties in the 10-member coalition remain committed to it.

President Cyril Ramaphosa's spokesman Vincent Magwenya also threw down the gauntlet to the DA, saying: "You can't be part of a government whose budget you opposed."

The DA found itself voting alongside South Africa's two biggest, and most populist, opposition parties - former President Jacob Zuma's uMkhonto weSizwe (MK) party and Julius Malema's Economic Freedom Fighters (EFF).

Advocating the nationalisation of key sectors of the economy, these two parties are the implacable foes of the pro-business DA.

But the three parties were united in opposing a VAT increase, believing it would hit the poor hard.

As DA leader John Steenhuisen put it: "The ANC is out of touch with the people, and if they bought their own groceries or filled their own tanks, they would know how expensive life already is."

But the ANC argued that a VAT increase - set at 0.5% this year and a further 0.5% next year - was necessary to raise revenue, and to offer public services such as health and education.

Crucially, the Inkatha Freedom Party (IFP) voted with the ANC, signalling the end of the alliance it formed with the DA in the build-up to the election in a failed bid to keep Ramaphosa's party out of power.

ActionSA - a small opposition party which broke away from the DA - helped the ANC clinch the vote.

It said it had reached a deal with the ANC that would see the VAT increase scrapped, while alternative ways of raising revenue for the government are explored.

"Yesterday's [Wednesday's] adoption of the report on the fiscal framework was merely one step in a multi-stage budgeting process before the final budget is approved," ActionSA said in a statement.

Prof Gumede said the ANC would find it difficult to convince the public to pay more taxes when public services were crumbling.

"The optics don't look good for the ANC," he told the BBC.

"The DA has taken the budget as an opportunity to make a big impact, and to show it is pro-poor."

The dispute over the budget is the latest sign of the sharp differences between the two parties, with the DA also challenging in the courts three other pieces of legislation - including the land expropriation act.

This law was one of the issues that led to US President Donald Trump's administration cutting aid to South Africa.

The Trump administration has now imposed tariffs of 30% on all South African imports, in a move that is likely to be a huge blow to its already floundering economy.

"They have got some bad things going on in South Africa. You know, we are paying them billions of dollars, and we cut the funding because a lot of bad things are happening in South Africa," the US president said, before going on to name other countries.

In a statement, Ramaphosa's office condemned the new tariffs as "punitive", saying they could "serve as a barrier to trade and shared prosperity".

But for many South Africans, the tariffs signal the need for the two biggest parties to resolve their differences and work together - or risk seeing the nation sink into a deeper economic crisis at a time when the unemployment rate is already at more than 30%.