The arrest of Machar last week was seen by some observers as the spark that could reignite the civil war that the rivals fought between 2013 and 2018 that killed some 400,000 people.
But others argue that the situation has changed considerably since the last conflict with new political struggles now centre-stage.
Succession
Analysts say Kiir, 73, is struggling with health issues and has been seeking to ensure his succession and sideline Machar politically.
"The succession is the major issue in South Sudan, with President Kiir not in good health and people within his party and government trying to take over," a Juba-based humanitarian told AFP, requesting anonymity for security reasons.
Since February, more than 20 of Machar's political and military allies have been arrested, some held incommunicado.
In at least three states, governors loyal to Machar have been replaced by Kiir's allies -- a move that violates the 2018 power-sharing agreement that ended the civil war.
Daniel Akech, a researcher at the International Crisis Group, fears Kiir's strategy amounts to "escalation after escalation", and that Machar's arrest marks a dangerous step towards war.
But the common framing of a struggle between Kiir and Machar may be outdated, said a Juba-based diplomat, who also spoke on condition of anonymity.
Machar has lost much popular support since joining the establishment in 2018, he said.
Many within Kiir's own party are angry that he appears to be positioning his former financial advisor, Benjamin Bol Mel, as his chosen successor, having made him a vice president earlier this year.
Rising conflict
The tensions have spilt over into conflict in several areas, often with an ethnic element.
In northeastern Upper Nile State, a militia of youths mainly from the Nuer community, known as the White Army, overran a military base in early March.
The military responded by shelling nearby areas, killing 20 people, mostly women and children, according to an administrative commissioner in Nasir County.
The fighting has already displaced more than 60,000 people, according to the UN.
Clashes have occurred in other areas, including airstrikes that have targeted Machar's forces near the capital Juba.
Money and soldiers
South Sudan, already one of the world's poorest nations, has faced a financial crisis over the past year after the war in neighbouring Sudan ended oil exports through one of its pipelines.
Oil accounts for up to 90 percent of government revenue. Production plunged from 140,000 to 20,000 barrels per day, the diplomat said.
After nearly a year without salaries, South Sudanese civil servants are excited following the country's resumption of oil exports.
The loss of funds means soldiers have gone largely unpaid for over a year, beyond the recent dispersal of a few months' wages.
The arrival of Ugandan forces in early March to support Kiir fuelled further discontent after rumours they were being paid in dollars, the diplomat added.
There has been only limited progress towards unifying the armies of Kiir and Machar, as mandated under the 2018 peace deal.
The widespread discontent could, Akech warns, lead some armed groups to break away from the control of the traditional leaders, leading to "decentralised violence".
International community
The UN has repeatedly called on both sides to uphold the peace process and Western embassies in Juba have offered to mediate.
But there is scant international attention being paid to the world's youngest nation.
The rising tensions have already led Germany and Norway to shut their embassies.
A former Kenyan prime minister, Raila Odinga, flew to Juba on Friday to help "de-escalate" tensions and met with Kiir, but was prevented from meeting with Machar.
Meanwhile, the tensions have hampered the fight against cholera at a time when aid agencies say the country is experiencing its worst epidemic in 20 years, with more than 40,000 cases and almost 700 deaths in six months.
