South Sudan's army will step up the fight against rebels with imminent strikes in Bor and Bentiu, the army spokesman told Al Jazeera.
The United States military says it is repositioning forces in Africa in preparation for possible further evacuations as the United Nations warned that the planned strikes on Monday could stall peace efforts and further escalate violence that has brought the country to the verge of civil war.
US envoy to South Sudan Donald Booth also said to Reuters, he had spoken to President Salva Kiir who said he is committed to talks with rebel leader Riek Machar to end the crisis without preconditions, as soon as his counterpart is willing.
However South Sudan spokesman Philip Aguer said on Monday, "the army is on its way to Jonglei and Unity states to retake territory and will attack within a few hours."
Never has there been a greater time of need in South Sudan.
Aguer denied that the conflict is stoked by ethnic tensions insisting the situation is political. He said about 2,000 soldiers have defected from government forces.
Pentagon spokesman Colonel Steve Warren told Reuters, "The combatant commander is repositioning his forces in the region to ensure that we've got capabilities necessary to respond to any request from the State Department."
Meanwhile, UN Secretary-General Ban Ki-moon said he will recommend to the Security Council on Monday an increase in peacekeepers of 5,000, to about 11,800 from 6,800, to better protect civilians, Reuters reported.
The UN estimates that at least 1,000 people have been killed and more than 100,000 are internally displaced, since fighting broke out over a week ago after President Kiir accused his former deputy, Machar, of attempting a coup.
Machar denied this allegation but told Al Jazeera on Sunday that he wanted to be the next leader of the country and run for president at the next election in 2015. He called on Kiir to step down.
Machar told Reuters on Monday that forces loyal to him now control oil fields in Unity and Upper Nile states, but he wants production to continue, saying funds from oil should be deposited in an escrow account so the country did not lose revenues due to the fighting.
He also said he is ready for dialogue to end the conflict once his detained allies are freed.
South Sudan's information minister Michael Makuei Lueth denied Machar's claims that he now controls the main oil fields.
He also said the government will not accept a rebel demand for politicians detained by the authorities over a "foiled coup" to be freed for peace talks to start.
Fighting continues to spread
Speaking from Juba Al Jazeera's Haru Mutasa said on Monday there is new fighting in Upper Nile state, another oil rich area, with sustained fighting continuing in Jonglei and Unity states.
The three states contain the country's oil fields, and two are along the northern border which could prompt the conflict to spill over into its neighbour, Sudan.
|Amadou Sy, senior fellow at the Brookings Institution, talks about fears of escalating violence.
The fighting has forced locals and foreigners alike to take cover at UN bases which are struggling to cope with the large influx.
The situation in the UN camps is getting volatile, and what started as a power struggle is now taking its toll on civilians, added Al Jazeera's correspondent.
"We had people with us that were South Sudanese but from a different tribe from the majority in the UN camp in Juba. People got angry that this particular tribe were with us, they said get them out or we will kill them, we had to leave," she added.
Stalled peace efforts
Toby Lanzer, the UN's humanitarian coordinator told Al Jazeera on Monday he expects hundreds of thousands of people are affected by the fighting, adding that no community in the country has been spared.
He said that it is now not only political parties involved, with armed youth increasingly taking matters into their own hands, particularly in different parts of Jonglei state.
"Everything must be done at the political level, the continued violence threatens peace efforts.
"There are about 50,000 people in UN bases, but there are also people hiding in churches, cathedrals, in between villages - wherever they feel safe," he added.
He urged financial assistance from the international community saying they couldn't afford any delay from donors.
"Never has there been a greater time of need in South Sudan," he added.