By Denis Dumo

JUBA, Sept 27 (Reuters) - South Sudan's government urged Sudan and regional states on Tuesday not to let opposition leader Riek Machar launch a new rebellion, after he threatened a return to the battlefield unless demands needed to revive a peace deal were met.

Machar is in the Sudanese capital Khartoum, having fled South Sudan after fighting flared between his supporters and those of his rival President Salva Kiir in July. The two had signed a shaky peace deal in 2015 after two years of conflict.

After talks in Khartoum involving senior commanders and officials in the SPLM-In-Opposition (SPLM-IO) last week, Machar and his allies in a statement ordered their forces to reorganise for "armed resistance" to Kiir's government.

"If the peace agreement can be revived then we can go back to Juba, but if not then armed resistance is an option," Machar's spokesman James Gatdet Dak told Reuters in Nairobi, adding the government must reappoint Machar, let more troops return to Juba with Machar, allow the swift deployment of a regional protection force and scrap decisions taken since July.

The growing tensions threaten to plunge South Sudan, which won independence from Sudan in 2011, back into full-blown conflict.

The regional grouping IGAD, which includes Sudan, has told both sides to halt the violence and aims to send a protection force to secure the peace, a move that has U.N. backing.

"We are appealing to all IGAD member state not to allow Riek Machar and his group to use their soil, including Sudan, to stage attacks and as a springboard for organising violence against the people and transitional government," deputy government spokesman Akol Paul Kordi told Reuters.

Pointing to splits in the opposition, Kordi said the SPLM-IO faction in Juba had sacked Machar and the former vice president had no right to make demands on the Juba government from Sudan as "he is in Khartoum as a private citizen."

Sudanese Foreign Minister Ibrahim Ghandour said his government would not allow the armed opposition to attack South Sudan from its territory, according to a report by the Paris-based Sudan Tribune news website.

Machar's statement had said the transitional government and peace deal had "collapsed", condemned the appointment of Taban Deng Gai as First Vice President to replace Machar and demanded the rapid deployment of the U.N.-backed protection force.

The statement called for the reorganisation of the SPLM-IO "so that it can wage a popular armed resistance" against Kiir.

(Writing and additional reporting by Edmund Blair in Nairobi; Editing by Raissa Kasolowsky)