By Mark Doyle
World Affairs Correspondent, BBC News
Aid agencies fear a new humanitarian disaster in DR Congo
The United Nations says some 2,000 Rwandan troops have entered the east of the Democratic Republic of Congo, where rebels opposed to the Rwandan government have been based for many years.
A UN spokesman said the Rwandan move was part of a peace plan agreed with the Congolese government.
However, the presence of Rwandan troops inside Congo is bound to spark controversy and could have a powerful impact across central Africa.
Rwanda has been described by some as the Israel of Africa.
The ethnic Tutsis of Rwanda experienced their genocide in 1994 but a Tutsi-dominated government then came to power and has ruled ever since.
Like the Israelis, the Tutsis have enemies on their borders, and now they have sent in their powerful army to deal with the ones who have bases in neighbouring DR Congo.
The parallels with Israel are not exact, of course.
Rwanda and Congo have been fighting what amounts to a proxy war: Rwanda accuses Congo of harbouring ethnic Hutu militias who took part in the 1994 genocide and Congo accuses Rwanda of backing a Congolese Tutsi separatist movement.
RWANDA IN DR CONGO
1994: After Rwanda genocide, Hutu rebels move to DR Congo (then Zaire)
1997: Rwanda invades DR Congo to deter cross-border rebel raids
1998: Rwanda invades again. At least four other countries intervene
2002: Rwandan military leaves DR Congo
2003: "Africa's world war" declared over
2008:Congolese Tutsi rebel offensive on Hutus
2009: Rwandan military enters DR Congo
The two countries recently agreed to work together militarily to deal with the Hutu side of this equation, and the march of the Rwandan army into eastern Congo can be seen in this context.
So there is the potential for this military move to be a step towards peace. But its fraught with danger.
Recent fighting in eastern Congo, between Congolese, left a quarter of a million people displaced.
Most of them will be terrified of the Rwandan presence and a new humanitarian disaster is possible.
And of course the Rwandan Hutu militias, a well-drilled military machine, are unlikely to give up to the Tutsi-dominated Rwandan army without a fight.
So there is also a very real possibility of Rwandans fighting Rwandans on Congolese soil.