South Sudanese rebels have shelled government positions in the oil-rich town of Bentiu, a day after the UN launched a $1.8bn aid appeal to stave off famine in the war-wracked country.

The spokesman for the country's military, the Sudan People's Liberation Army (SPLA), told Al Jazeera that rebels backing former vice president, Riek Machar, destroyed a mounted vehicle and captured a tank in Bentiu, near the border with Sudan.

Philip Aguer said the SPLA was pursuing the rebels, calling Tuesday's attack a clear violation of an agreement signed earlier this month that called for an end to hostilities.

Defence Minister Kuol Manyang told the AFP news agency that "the rebels are shelling our positions in Bentiu," adding that the military would "act in self defence".

Bentiu has been hotly contested between the two sides, with control of the town changing hands several times.

South Sudan has been plagued by violence since December 2013 when President Salva Kiir accused his sacked deputy Machar of attempting a coup.

Fighting broke out in the capital Juba, setting off a cycle of retaliatory massacres across the country.

Much of the violence pitted the ethnic Dinkas, who back Kiir, against the ethnic Nuer, who support Machar.

'Brink of famine'

The EU has set Kiir and Machar a March 5 deadline to strike a final peace agreement, but previous deadlines have been repeatedly ignored despite the threat of sanctions.

Tuesday's clashes come a day after donors pledged $529m towards a $1.8bn aid appeal by the UN, with more than 2.5 million people on the brink of famine.

Anne C Richard, the US Assistant Secretary of State for Refugees, said some areas were "teetering on the brink of famine," and that people "continue to suffer and die unnecessarily because their leaders are unwilling to do what it takes to restore peace".

Almost two million have been forced from their homes and 500,000 of them have fled abroad to neighbouring countries.

According to the International Crisis Group at least 50,000 people have been killed in the conflict.

Both sides have been accused of war crimes, including mass killings, rape, attacks on hospitals and places of worship, and recruiting child soldiers.