By Denis Dumo

JUBA, April 22 (Reuters) - Fighting in South Sudan's oil-producing Upper Nile State in the past two days has killed at least 38 and the toll could rise, a military spokesman said on Wednesday, while aid groups said they may be forced to trim operations.

Thousands of people have been killed and more than a million have fled their homes since fighting between supporters of President Salva Kiir and former vice president Riek Machar erupted in the world's newest nation in December 2013.

Several ceasefires have been agreed and broken and each side accuses the other of violating one announced in early February.

Army Spokesman Colonel Philip Aguer said in the first incident, South Sudan People's Liberation Army (SPLA) soldiers fought rebel forces allied to Machar outside Malakal, killing 36 of them and repulsing the rest.

Aguer said in a second incident, fighting between guards protecting the governor of Upper Nile and soldiers under Johnson Olony, a Major General in the South Sudan People's Liberation Army (SPLA) from the Shilluk tribe in Upper Nile, killed two.

Aguer said the situation had calmed down. But Médecins Sans Frontières (MSF) said in a statement:

"At this moment the situation in Malakal town remains extremely volatile and it is expected that confrontations will continue in the coming hours.

"MSF teams have today been forced to suspend some...outreach activities to remote areas hosting internally displaced people."

It said that in April alone, 4,600 people had been displaced by fighting in Malakal.

The World Food Programme said three of its workers have been missing since April 1, when they were travelling in a convoy carrying food aid that encountered inter-communal fighting between Malakal to Melut.

WFP said another WFP worker who was abducted at gunpoint in Malakal airport had still not been found.

"Because of increasing concerns about staff safety, WFP is re-assessing its ability to work in some parts of Upper Nile State," it said in a statement."

"The agency plans to temporarily reduce its operations in those areas where it no longer believes it's safe to work." (Reporting by Denis Dumo; Additional reporting and writing by George Obulutsa; editing by Ralph Boulton)