Guehenno said he would travel to Brussels next week for discussions with EU officials on protecting refugees and people fleeing villages in eastern Chad because of the fighting in Sudan's Darfur region.
"Genocide is a crime against humanity as a whole, not just against it's immediate targets. It therefore falls on the world at large to act."
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He stressed that talks on the joint force were still at a preliminary stage.
UN security council diplomats said the hope was that the EU would send forces into Chad for about six months, to be replaced after that by a UN operation.
It is far from certain whether the UN, which is struggling to find more than 20,000 troops and police for Darfur, could field a force in Chad and the EU only wants its troops to stay for a limited period.
Guehenno said the proposed peacekeeping mission would help specialised Chadian forces ensure security in camps for refugees and internally displaced persons in the area.
He stressed that there would have to be close "co-ordination mechanisms" between such a EU-UN force and the planned African Union-UN force that is to take over peacekeeping from underfunded and poorly equipped AU troops in Darfur.
Last month, France asked the EU to send up to 12,000 troops to Chad to set up a humanitarian corridor for Darfur refugees.
Chad has repeatedly called for international assistance as it struggles to protect the large number of refugees arriving in the country and deal with a domestic rebellion but until recently has balked at a military force.
Sudan and Chad accuse each other of supporting rebel forces in their respective territories.
The northern half of the Central African Republic has recently been hit by a flare-up of violence by bandits and armed opponents of Francois Bozize, the president.
Bozize's opponents accuse him of mismanaging public funds and dividing the nation.
A senior commander from the Popular Army for the Restoration of Democracy, or APRD, the largest armed group in the country and primarily active in the northwest, says talk of a ceasefire may be premature.