A BBC correspondent at a UN base outside the town has heard heavy firing and seen government troops and armoured vehicles driving towards the centre.
Rebel forces deny UN charges that they killed hundreds of people along ethnic lines after seizing the town in April.
Bentiu, in the north, has changed hands several times since fighting broke out in South Sudan last December.
Tensions came to a head after President Salva Kiir accused his sacked deputy, Riek Machar, of plotting a coup.
Mr Machar denied the charges, but then mobilised a rebel force to fight the government.
The government offensive in Bentiu, capital of Unity State, comes two days after President Kiir told the US Secretary of State John Kerry that he was prepared to hold direct peace talks with Mr Machar.
Gunfire could be heard in the distance throughout the morning, says the BBC’s Alastair Leithead, who is at the UN Mission in South Sudan (UNMISS) compound on the outskirts of the city.
A long line of government troops in armoured personnel carriers, heavily armed, are moving slowly towards the city, he adds.
On Friday, Mr Kerry secured an agreement from President Kiir to meet with Mr Machar for talks in Addis Ababa that would be mediated by Ethiopian Prime Minister Hailemariam Desalegn.