African powers trying to broker a peace deal in the world's newest country, South Sudan, have said its government has committed to a ceasefire after two weeks of clashes that have caused more than 120,000 people to flee.

The president, Salva Kiir, agreed to an "immediate cessation of hostilities", according to east African leaders mediating in the crisis. But they added that his rival, Riek Machar, whom Kiir accuses of trying to mount a coup two weeks ago, had not made the same commitment.

The mediators "welcome the commitment by the government of Republic of South Sudan to an immediate cessation of hostilities and call upon Dr Riek Machar and other parties to make similar commitments", they said in a statement.

At least 121,600 people have fled their homes in South Sudan but the total number is likely to be much higher, according to the UN, which has urgently requested $166m (£100m) from donors to deal with the humanitarian crisis.

The number of internally displaced people in the capital, Juba, has reached an estimated 25,000 since the power struggle erupted in mid-December. About 63,000 people have sought refuge at UN peacekeeping bases, mainly in Juba, Bor, Malakal, Bentiu and Pariang.

Toby Lanzer, the UN humanitarian co-ordinator in South Sudan, said: "This is an extremely difficult time for the people of this new nation, and it is crucial that aid agencies have the resources they need to save lives in the coming months.

"There are at least 90,000 people who have been displaced in the past 10 days."

Lanzer has estimated that the death toll has already surpassed 1,000.

UN officials are particularly worried about those in and around the town of Bor, in Jonglei state, where fighting has been especially intense. Some UN officials have returned to Bor after all aid workers were evacuated on 23 December to assess conditions for the 15,000 people who fled to the UN base.

Fighting has persisted for four days in the oil-producing northerly region of South Sudan. Government forces said they had finally defeated Machar's rebels in Malakal, capital of Upper Nile State. The claim could not be independently verified.

Outside UN bases, there are large groups of displaced people in Jonglei, Lakes, Warrap and Unity states. An estimated 45,000 people are in Awerial county in Lakes state, but aid agencies have been unable to reach this large group because of security fears.

Agencies have requested the $166m from now until March 2014 to help meet the immediate needs of people affected by the crisis. This includes emergency programmes for some 200,000 refugees from Sudan.

"In Bor and Bentiu this week, I have seen just how badly the communities caught in violence need our help," Lanzer said. "Our priorities are to stay, protect, and deliver. I hope that donors and compassionate people around the world act swiftly to give aid agencies the required resources to help the people of South Sudan at this critical juncture."

The $166m represents the most urgently required resources from the overall $1.1bn programme set out by the aid community for 2014 in South Sudan.

The head of the UN mission in South Sudan, Hilde Johnson, has said that well over 1,000 people have been killed since the start of the violence on 15 December and that the casualty figures are likely to rise.