KHARTOUM - Ugandan President Yoweri Museveni said Monday that leaders of Inter-Governmental Authority for Development would join hands together against forces allied to South Sudanese ex-vice president Riek Machar, if he fails to respond to a ceasefire agreement offered by the organization, the Sudan tribune reported.

The regional body would "defeat Machar" to avert further escalation of violence in the new nation, according to the report.

"We gave him (Machar) four days and if he does not (comply with the agreement), then we shall have to go for him. That is what we agreed on," Museveni said in the South Sudanese capital of Juba.

The South Sudanese government said earlier that it agreed on the ceasefire, but Machar rejected it, demanding a mechanism to monitor the ceasefire.

During the past two weeks, South Sudan has witnessed clashes between two military factions, one descending from the Dinka tribe, to which South Sudanese President Salva Kiir Mayardit belongs, and the other descending from the Nuer tribe, to which Machar belongs, who is accused of plotting to topple the government.

The clashes have left thousands of people dead, over 121,600 civilians displaced and some 63,000 others took refuge at various UN compounds around the country, according to UN reports.