(Adds U.S. condemning expulsion)

By Denis Dumo

JUBA, June 3 (Reuters) - South Sudan on Tuesday rejected a United Nations appeal to halt the planned expulsion of the world body's top humanitarian aid official in the country, saying he had regularly spoken out against the government.

U.N. Secretary-General Ban Ki-moon on Monday condemned the move to expel Toby Lanzer, deputy head of the U.N. Mission in South Sudan, and said the British-born envoy had been "instrumental in addressing the increasing humanitarian needs of conflict-affected communities" in South Sudan.

Presidential spokesman Ateny Wek Ateny told Reuters, "It is impossible to reverse ... (the) idea of expelling Toby Lanzer." The cabinet came to the decision after Lanzer made comments "completely against the government".

He gave no details. Lanzer has often posted messages on Twitter about the dire situation in a country that has been mired in conflict since late 2013.

Lanzer was due to leave shortly and his successor, Eugene Owusu of Ghana, has already been nominated.

More than 2 million people have fled their homes, with 555,000 departing for neighbouring states. About a third of the nation's 11 million people rely on food aid and other assistance.

"He (Lanzer) has echoed the views of many members of the international community who believe it is time that the leaders of South Sudan pay heed to the suffering of their people," the European Union delegation in South Sudan said in a statement.

Lanzer's expulsion was "an affront to the international community" that showed "a callous disregard for the suffering of the South Sudanese people," the U.S. State Department said in a statement on Tuesday.

Fighting has pitted soldiers backing President Salva Kiir, the country's leader since independence from Sudan in 2011, against those loyal to his former deputy Riek Machar, who was sacked from his post in mid-2013.

The conflict, which has flared up in recent months, tends to follow ethnic lines that divided southern tribes even before independence. Kiir is an ethnic Dinka and Machar is from the Nuer group.

Peace talks in Ethiopia, sponsored by African states and backed by the United States and other countries, have stalled over power sharing. Mediators have been trying to restart negotiations.

Kiir met visiting African mediators on Tuesday in Juba, including South African Vice-President Cyril Ramaphosa, as part of efforts to reunify the fractured ranks of his ruling SPLM party, Ateny said.

Kiir also met South Sudanese political opponents who were detained early in the crisis before being allowed to leave for Kenya. Of 10 detained, five returned on Monday as an advance party and the rest were due back on June 25, Ateny said. (Additional reporting by Tom Miles and Stephanie Nebehay in Geneva, and Eric Walsh in Washington; Writing by Edmund Blair; Editing by Tom Heneghan, Toni Reinhold)