Hizbul Islam, a coalition of groups fighting Somali government forces and African Union peacekeepers, has said it has rejected a ceasefire offer from Somalia's new president.
Muse Abdi Arale, a spokesman for the group, said on Sunday that reports of a ceasefire plan between his group and Somalia's government were "baseless," the AFP news agency reported.
"We will attack the enemy and their stooges anytime we want," Arale said.
Sharif Ahmed, the president of Somalia, announced on Saturday that he had agreed to proposals for a truce with the leading rebel group and had offered to accept the implementation of Sharia, or Islamic law, in the country.
"I met with religious leaders and elders and accepted their demand for ceasefire and reconciliation with the opposition members, and I call on all opposition parties to halt the unnecessary violence," Ahmed said.
Arale said his group had met with the mediators but only discussed plans for a partial withdrawal of rebel fighters.
Sunday's rejection comes a day after Omar Abdirahsid Sharmarke, the Somali prime minister, led the inaugural session of the new government in Mogadishu, Somalia's capital.
It also follows renewed heavy fighting in Mogadishu between opposition fighters and government and African Union forces.
At least 49 civilians were killed in the clashes in the capital, the independent Elman Human Rights Organisation has said.
Hizbul Islam is against the presence of AU troops in Somalia and has said it will battle them until they leave the country.