Rwanda-backed rebels who have captured eastern Congo’s largest city say they plan to take their rebellion to the capital and seek to gain political power.

The rebels said the ceasefire would start Tuesday. The announcement came shortly after the U.N. said 900 people were killed in last week’s fighting in Goma between rebels and Congolese forces.

Congolese forces were losing control of the city of Goma on Wednesday after rebel groups backed by Rwanda took over the airport and shared plans to set up a new administration.

M23 rebels in the Democratic Republic of Congo said they would begin a unilateral ceasefire on Tuesday as the Group of Seven nations demanded that the Rwanda-backed militia halt an offensive that has shaken the eastern part of the country.

The Rwanda-backed rebels who seized a major city in eastern Congo have declared a unilateral ceasefire, citing humanitarian grounds.