Heavy fighting involving tanks and helicopters raged in South Sudan between troops loyal to President Salva Kiir and those backing Vice President Riek Machar.
The violence, which erupted in the capital Juba five days ago, risks a return to civil war in the world’s newest nation.
“This is the time to massively reinforce UN action,” Ban told reporters, adding that fighting had spread to areas outside of Juba in Central Equatoria state.
“When a government cannot or will not protect its people, and when warring parties seem more intent on enriching and empowering themselves at the expense of their people, the international community has a responsibility to act,” he said.
Two Chinese UN peacekeepers and one local UN staff member have been killed in the fighting, Ban said.
UN peacekeepeers have been deployed in South Sudan since the country gained independence from Sudan in 2011.
“We desperately need attack helicopters and other material to fulfil our mandate to protect civilians,” Ban said.
“I also urge all countries contributing to (the UN mission) UNMISS to stand their ground. Any withdrawals would send precisely the wrong signal, in South Sudan and across the world.”
Kiir and Machar have long been rivals in politics and on the battlefield. A civil war that began in December 2013 came a few months after Kiir dismissed Machar as his deputy. The pair signed a peace deal in August 2015, but implementation has been slow.
“Yet again, the leaders of South Sudan have failed their people,” Ban said. “What kind of leadership is it that resorts to deadly weapons and identity politics, time and again? Failed leadership.”
In a rare move, Ban called for an immediate arms embargo on South Sudan and targeted sanctions on leaders and commanders blocking implementation of the peace deal.
The United States, Britain, France, Angola and other members of the Security Council pushed for an arms embargo on South Sudan earlier this year. But veto power Russia said it was opposed to such a move because it did not believe it would be helpful to implementation of the peace deal.
The president of South Sudan and his opposition rival called for a ceasefire.
President Salva Kiir declared a halt in fighting and raised fears of a return to civil war that could draw in even more of the East African country’s ethnic groups. On Saturday, the troubled nation marked the fifth anniversary of its independence from Sudan.