Juba - South Sudan accused Sudan on Sunday of air-dropping weapons to rebels, just as the presidents of the African neighbours were about to meet to finalise a border security deal to restart oil exports.
Sudan dismissed the charges and any links to rebels in the South, which seceded from Khartoum in July last year under a 2005 peace deal that ended decades of civil war.
Sudan, in turn, often accuses Juba of supporting rebels in its borderlands.
The claims came as Sudan's President Omar Hassan al-Bashir arrived in Ethiopia to wrap up with his southern counterpart, Salva Kiir, two weeks of talks to end hostilities.
African and Western officials have been trying to mediate a border security agreement between the rivals which came close to war in April.
South Sudan's army spokesperson Philip Aguer said Sudanese military aircraft parachuted eight parcels of weapons and ammunition to forces of militia leader David Yau Yau in the country's east on Friday and Saturday.
The rebels later attacked the town of Likuangole in Jonglei state, but were repulsed by the South's army (SPLA), he said.
"The planes dropped arms and ammunition around Likuangole in front of everybody, including Unmiss (the UN mission in South Sudan)," Aguer said.
"This is not in the spirit of the current talks in Addis Ababa.
"While Khartoum talks peace, their deeds on the ground tell a contrary story of hostile acts of conspiracy against the Republic of South Sudan."
Sudanese army spokesperson al-Sawarmi Khalid said: "The Sudanese army has no relation to any rebel group in South Sudan and is not giving any military support to these groups."
Yau Yau, one of several militia leaders fighting the government in South Sudan, is attempting to recruit armed youth from the Murle ethnic group since arriving in the area in late July, residents in Jonglei say.
Rights groups and Unmiss accuse South Sudan's army of human rights violations during a disarmament push aimed at ending a cycle of clashes between the Murle and Lou Nuer tribes.
Nearly 900 people died when about 7 000 armed youths of the Lou Nuer tribe attacked Murle villages in the Pibor area at the end of last year, according to the United Nations.
South Sudan is awash with weapons after a decades-long civil war with Khartoum that killed an estimated 2 million people.
The government, run mostly by former guerrilla fighters, has struggled to assert control over its vast and restive territories since declaring independence.