South Sudan's President Salva Kiir said Monday he was not opposed to the deployment of a strengthened UN force but expressed "very serious concerns" over the internationally-backed plan.
The comments come as the UN faces renewed criticism over repeated failures to protect civilians, including foreign aid workers.
In a speech at the opening of parliament, Kiir said more time and further negotiations were needed before the deployment of a new contingent with a stronger mandate.
"We need to be engaged in a discourse and exchange ideas on what is the best way forward, rather than be presented with a fait accompli from outsiders," Kiir said in the capital Juba.
On Friday, the UN Security Council approved a US-drafted resolution backed by regional bloc IGAD to strengthen the 12,000-person peacekeeping mission, known as UNMISS, with 4,000 additional troops drawn from regional armies and equipped with a more aggressive mandate.
But it also threatened an arms embargo if South Sudan throws up barriers to its deployment.
The latest allegations about UN failings emerged on Monday in a Human Rights Watch (HRW) report which said peacekeepers did nothing as women were raped, civilians murdered and foreign aid workers targeted during a flareup last month in which hundreds were killed in Juba.
In one incident, government soldiers rampaged through a hotel compound "where they killed a prominent journalist, raped or gang raped several international and national staff of (aid) organisations, and destroyed, and extensively looted property," the group said.
During the hours-long assault the UN "did not respond to direct calls for protection by aid workers at the... compound, a kilometre (half a mile) from their base," HRW said.
The hope is that a strengthened UN force might prevent a repeat of such atrocities.
Kiir said he was "not instinctively or automatically" against the UN-mandated force.
But he insisted that sovereignty came first, winning cheers from the assembled MPs in a parliament dominated by his loyalists.
"We welcome assistance, we are attentive to advice. But assistance requires dialogue, it should not turn into an imposition that becomes an intervention, in which our sovereignty is compromised and our ability to govern effectively diminishes," Kiir said.
South Sudan's civil war began in December 2013 when Kiir accused his former deputy Riek Machar of plotting a coup. The fighting has split the country along ethnic lines and driven it to the brink of collapse.
A peace deal signed between the government and rebels almost a year ago has so far failed to end the conflict and last month, Juba was rocked by several days of heavy fighting between Kiir's forces and those loyal to Machar.
In a statement Monday, Machar's supporters said the UN resolution creates "a conducive security environment that will facilitate the smooth implementation" of the peace deal.
But the statement also highlighted entrenched suspicions, describing the recent fighting as a "return to armed conflict" and accusing Kiir of engineering the "overthrow" of the unity government which was formed in April.
In other remarks, Kiir -- a former guerrilla commander -- also promised to reform the military and make it "completely subordinate to the authority of a civilian government."
Without offering any details about how the opposing factions might be reconciled, Kiir promised to create a single national army by May 2016.
"The unnatural reality of one country and two armies coexisting side by side is dysfunctional and already proven to be disastrous as we witnessed during July's tragic events," he said.
Kiir also promised to investigate alleged abuses by soldiers and called for more international financial assistance to help rebuild the economy.
Nearly a million refugees have now fled the brutal conflict and are suffering dire conditions in camps across the region, refugee agency UNHCR said on Monday.
"With refugees fleeing South Sudan in their thousands, surrounding countries are straining under the weight of large numbers of displaced people and critically underfunded operations," the UNHCR said in a statement.
"Already there are some 930,000 refugees in the region, and more are arriving daily."
It has only received a fifth of the $609 million (545 million euros) it needs for operations helping refugees now scattered across six countries, as well as the 1.6 million internally displaced, it added.