* Fighters from Sudan and Congo now involved, say rebels
* Rebels accuse army of destroying Machar's hometown
* Allegations raise political temperature ahead of talks (Recasts with rebel claims)
By Andrew Green
JUBA, Feb 2 (Reuters) - South Sudanese rebels accused government forces on Sunday of razing the hometown of their leader Riek Machar, violating a ceasefire, and said the army was drawing support from foreign fighters now in the country.
Rebel spokesman Lul Ruai Koang said government SPLA forces and fighters from the Sudanese Justice and Equality Movement - a rebel group from north of the border - had destroyed the northern town of Leer on Saturday, massacring women and children as they fled.
An army spokesman said he had not received any reports of fighting in Leer, where the medical charity Medecins Sans Frontieres (MSF) said last week more than 200 of its staff had been forced to flee because of growing insecurity.
The government accuses the rebels of flouting the ceasefire signed on Jan. 23.
The claims and counter-claims came as east African ceasefire monitors began to arrive in South Sudan, seven weeks after violence erupted in the capital, Juba, before spreading across the world's newest state.
"(President Salva) Kiir's forces burned down the whole of Leer town and entire surrounding villages," Koang said in a statement.
"The latest destruction of Leer town in Unity state has no strategic, operational or tactical importance, but mere need for psychological satisfaction."
Koang said the Ugandan military, which gave air and ground support to the SPLA as it battled to recapture rebel-held towns before the ceasefire, had swollen its ranks with fighters from the defeated M23 Congolese rebel group.
Hundreds of M23 rebels fled into Uganda after the Congolese army and a U.N. brigade flushed them from their strongholds. SPLA spokesman Philip Aguer said he had received no reports of foreign militiamen joining the conflict.
Ugandan army spokesman Colonel Paddy Ankunda called the rebel allegations "cheap lies".
Thousands of people have been killed and more than 800,000 have fled their homes since fighting was triggered by a power struggle between President Kiir and Machar, his former deputy whom he sacked in July.
The conflict, which has taken on a largely ethnic dimension between the Dinka and Nuer tribes of Kiir and Machar respectively, has brought oil-producing South Sudan, a country the size of France, to the brink of civil war.
Machar on Friday accused Kiir of sabotaging the peace talks - which resume in neighbouring Ethiopia this week - and of waging a campaign of ethnic cleansing, in a Reuters interview at his bush hideout in remote Jonglei state.
An advance team of monitors sent by east African nations arrived in Juba on Sunday to start observing the shaky truce.
Diplomats expect them to focus on the three flashpoint towns of Malakal and Bentiu, near the main oilfields, and Bor, where some of the heaviest clashes have occurred, as well as the capital.
"We will start our mission, at least the teams will be deployed, within the next week," General Gebreegzabher Mebrahtu, a retired Ethiopian general who is leading the advance team, told reporters in Juba.
The violence, the worst since South Sudan won independence from Sudan in 2011, has caused a humanitarian crisis.
At least 3.2 million people - more than a quarter of the population - face food shortages, the United Nations says. Aid agencies say insecurity is hampering their operations. (Additional reporting by Richard Lough in Nairobi, Aaron Maasho in Addis Ababa and Elias Biryabarema in Kampala; Writing by Richard Lough; Editing by Robin Pomeroy)