By Denis Dumo

NAIROBI, Sept 10 (Reuters) - South Sudan's parliament unanimously voted on Thursday to adopt a peace deal agreed last month by President Salva Kiir and rebel leader Riek Machar, amid mounting pressure for both sides to lay down their arms.

The world's newest country has been ravaged by war since December 2013, when soldiers loyal to Kiir clashed in the capital Juba with troops loyal to his former deputy, Machar.

Kiir signed the deal on Aug. 27 but complained about the details and said he had faced intimidation from other nations prior to signing. Sporadic fighting has continued since then, with both sides blaming each other for ceasefire violations.

The clashes have prompted the United Nations Security Council to warn both sides that it could impose sanctions if the August deal collapses.

"There was no fighting today but the rebels are mobilizing to attack Malakal anytime soon," said Philip Aguer, South Sudan's military spokesman, referring to the capital of the oil-rich Upper Nile State. Rebels said government troops had attacked their positions in the same state on Monday.

Manasseh Magok Rundial, speaker of the parliament, said it had backed the agreement in an extraordinary sitting.

"This is your resolution and decision," he told lawmakers. (Writing by Drazen Jorgic; Editing by Mark Trevelyan)