The US Secretary of State, Condoleezza Rice, is in the Ethiopian capital, Addis Ababa, for a series of meetings on some of Africa's worst conflicts.
Ms Rice says the US cannot let war return to South Sudan
Ms Rice is to discuss the situation in the Democratic Republic of Congo, Somalia and Sudan with regional presidents and ministers.
However, none of the three countries' leaders are going to Ethiopia.
She said she was "increasingly concerned about several crisis spots in Africa" before starting the talks.
She will also hold bilateral talks with Ethiopian Prime Minister Meles Zenawi.
They are expected to discuss renewed tensions with neighbouring Eritrea.
A deadline set by an international border commission for the countries to demarcate their shared border expired on Friday without agreement.
Her first talks were on the latest fighting in DR Congo, with the presidents of Rwanda, Burundi and Uganda, as well as Congolese ministers.
They agreed to strengthen the Congolese security institutions, Ms Rice said after the talks.
All countries agreed not to "harbour negative forces" - a reference to Rwandan Hutu rebels, which are at the heart of fighting across the region.
The Congolese army, backed up by UN peacekeepers, is also trying to disarm a dissident Tutsi general.
Laurent Nkunda refuses to join the army, saying his men must keep their arms to defend themselves against the Hutus.
Rwanda denies claims it is backing Gen Nkunda.
As President Abdullahi Yusuf is in hospital in Kenya, Ms Rice is due to discuss the Somali crisis with his newly appointed Prime Minister Nur Hassan Hussein, as well as Mr Meles.
The US supported Ethiopia's intervention in Somalia last year, to help the government oust Islamists from Mogadishu.
However, Ethiopia's intervention is unpopular in Somalia and insurgents continue to stage attacks.
The UN says that one million Somalis have been displaced by the fighting, including 60% of the capital's residents.
Mr Meles says he wants to withdraw his forces but cannot until they are replaced by a promised African Union peacekeeping force.
The BBC's Elizabeth Blunt in Addis Ababa says the US is offering to help African countries who are willing to send troops to Somalia as part of a peacekeeping force.
"We do believe that peacekeeping efforts need to take place in Somalia," she said.
"We appreciate very much the [Ugandan] grounding forces that are there. They frankly need to be joined soon by other forces."
On Sudan, Ms Rice is due to discuss the tensions in the US-brokered 2005 peace deal between north and south.
"That is really an agreement that we cannot afford to let unravel," she said.
She is also expected to discuss the conflict in Darfur with African officials.
However, Sudan's government has signalled that it will not meet Ms Rice, reports the AP news agency.