* Government, rebels accuse other of flouting ceasefire
* United Nations wary of conflict spilling over borders
* Pressure mounting on Juba to free remaining detainees
By Aaron Maasho and Edmund Blair
ADDIS ABABA, Jan 31 A regional African group mediating to end the conflict in South Sudan aims to deploy the first monitors of a shaky ceasefire at the weekend, senior officials said on Friday.
The IGAD group also joined international calls for the release of the last four of a group of detainees in South Sudan whom the government is investigating over charges they plotted a coup, but whose continued detention could hinder peace talks.
The government of President Salva Kiir and rebels loyal to his sacked deputy Riek Machar agreed a ceasefire deal on Jan. 23, but both sides have accused each other of violations. Analysts say Machar does not have full control of rebel forces.
Thousands of people have been killed and more than half a million people have fled their homes since fighting erupted in mid-December.
The conflict has brought oil-producing South Sudan, which declared independence from Sudan in 2011, to the brink of civil war. Regional and world powers worry about the potential for a spillover of violence in an already volatile region of Africa.
IGAD heads of state called for "an initial monitoring and verification mechanism presence in South Sudan within 48 hours", mediators from the East African grouping said after they met on Friday on the sidelines of an African Union summit in Addis Ababa.
That 48-hour timeline would give the group until Sunday to put the advanced party of monitors on the ground.
"The deadline of 48 hours which the chief mediator mentioned ... is for us absolutely real," Tim Morris, Britain's special envoy to South Sudan, told Reuters.
IGAD's efforts are being supported by the United States, Britain and Norway, three of South's Sudan's biggest donors.
Morris said U.S., British and Norwegian representatives might join what could be a 30-strong team of verification experts. He said China might also take part.
The British envoy said the monitors would focus on four flashpoint towns: Juba, the capital where fighting first erupted; Malakal and Bentiu, both in northern oil-producing areas; and Bor, scene of some of the heaviest fighting.
The U.N. mission in South Sudan, UNMISS, whose bases have taken in civilians fleeing fighting, was also ready to help monitoring if requested, U.N. Deputy Secretary-General Jan Eliasson told Reuters in Addis Ababa.
"We have offered our assistance in this regard, if that is needed," he said at the AU headquarters in Ethiopia's capital.
When fighting erupted in mid-December, South Sudan detained 11 politicians it accused of plotting to overthrow Kiir's government, charges the rebels have denied. Supporters of Machar, who has led the campaign from the bush, have demanded all 11 be freed, but did not make it a condition for the ceasefire.
IGAD heads of state commended South Sudan for releasing seven of those held, and called on the government "to expedite the legal process of the remaining detainees."
Peace talks are due to resume around Feb. 7, but one South Sudanese official acknowledged they might struggle to make progress while the last four, who include the former secretary-general of Kiir's ruling SPLM, were still held.
Western powers have already said they should be freed. South Sudanese officials say they could be pardoned, but only once the Justice Ministry's investigation is completed.
Adding his voice to calls for freeing the four, Eliasson also urged regional nations to help in the "de-escalation" of the crisis. He did not mention any countries, but Uganda's military support for Kiir's forces has worried diplomats.
"It is very important that this conflict doesn't turn into a regional conflict," Eliasson said. (Writing by Edmund Blair; Editing by Richard Lough and Mark Trevelyan)