The United Nations said it was being denied access to strife-hit parts of the country
"The United Nations Mission in South Sudan (UNMISS) is extremely concerned over increased reports of violence and armed conflict in various parts of the country in the last few weeks," a statement said, mentioning fighting in the northern Unity State and the southern Equatorias.
The UN said it was being denied access to strife-hit parts of the country and "condemns in no uncertain terms these acts of violence and attacks against non-combatant civilians."
UNMISS also said it had been denied access to an area where around 21 civilians were reportedly killed in an ambush on the road between Juba and the southern city of Yei over the weekend.
Also on Wednesday information minister Michael Makuei held a press conference to deny rumours of Kiir's death that had circulated in recent days, triggering fear and tension in the capital Juba.
"This is a mere lie, there is nothing as such, Salva Kiir has not even been sick," said Makuei, slamming "wild rumours" he said aimed to divide the people of South Sudan.
Later in the day Kiir, 65, was driven about town in the back of a pick-up to prove he was alive.
In recent days residents of Juba had reported a higher than usual presence of soldiers on the streets, as the rumours coincided with mounting concerns over an uptick in violence in the troubled nation.
"We are scared of the situation. You cannot know what is exactly happening," said Moses Modi, a resident of Juba who was staying in his house over security fears.
Another Juba resident, speaking on condition of anonymity, reported that some schools had sent pupils back home.
South Sudan, which gained independence in July 2011, descended into war just two and a half years later when Kiir in December 2013 accused his former deputy Riek Machar of plotting a coup.
Numerous attempts to shore up a fragile truce failed, and in a major setback to peace efforts, fierce clashes erupted in Juba on July 8 this year between Kiir's guards and troops loyal to Machar.
The international community has expressed deep concerns over a spread in violence since the July clashes, which pushed the number of refugees from the war-scarred nation past the one-million mark, according to the UNHCR.
In a further blow to peace hopes, Machar last month urged "a popular armed resistance" against his rival's government.
Machar, who fled to Khartoum in the July fighting, on Wednesday left for South Africa for medical tests.
An influential group of South Sudanese politicians known as the 'former detainees' after their arrest when war broke out in 2013, added their voices to the many raising concerns.
They said they were "greatly disturbed by the recent increase of war and violent conflict all over again; its ever deepening intensity and level of brutality; an apparently all-pervasive and creeping sense of resentment and hate".
"The country is on the edge of a precipice," the group said.