South Sudan's parliament voted on Thursday to extend President Salva Kiir's term in office until 2021, a move likely to undermine peace talks as opposition groups have said the change would be illegal.

"Now the speaker hereby declares that the transitional constitution amendment ... is hereby passed by (the) national legislature," Anthony Lino Makana, speaker of parliament, said.

Lawmaker Paul Youani Bonju, chairperson of parliament's information committee, said the extension, which will also apply to vice presidents, state legislatures and governors, would bolster the government negotiating team in ongoing peace talks with rebel groups in Khartoum, Sudan.

South Sudan, the world's youngest nation after gaining independence from Sudan in 2011, erupted into violence in late 2013 over a political disagreement between Kiir and his former vice president Riek Machar.

Machar's rebel group reiterated its opposition to extending officials' terms.

"We regret the move as it shows the regime is playing games at the negotiating table. The international community should not recognize this move and the regimes should be declared as a rogue regime," Mabior Garang de Mabior, spokesperson of SPLM- IO, told Reuters by phone from Nairobi.

The law change will extend the presidential and other officials' terms until July 12, 2021.

The government first submitted the proposed law change to parliament early this month.

This week, Machar's rebel group rejected plans to reinstate Machar as vice president, saying it had failed to dilute Kiir's strong power base.

The civil war has killed tens of thousands of people and forced millions to flee from their homes.

It has also cut South Sudan's crude oil production, which the government depends on for revenue, with output at less than half its pre-war level of 245 000 barrels per day.