Hizbul Islam has waged a three-year insurgency alongside Al-Qaeda-linked al Shabaab group, but the two have had their differences in southern Somalia.
The groups have held discussions on combining forces against Somalia's government, which is backed by foreign troops but whose authority the rebels have confined to a few blocks of the war-scarred capital Mogadishu.
"The dialogue on unification between Hizbul Islam and al Shabaab is still going on although it has halted at its third stage," Hizbul Islam leader Sheikh Hassan Dahir Aweys told Reuters.
"It is not so important to mention the reason for the current standstill or the bone of contention but I hope we shall reach comprehensive agreement, whether it takes a short while or a long time."
In the last few weeks, the rebels have stepped up their fight in the coastal capital Mogadishu in the hope of toppling the western-backed transitional administration, which is propped up by an African Union peacekeeping mission, AMISOM.
FOREIGN FORCE
The AU wants to increase its force in the lawless nation to 20,000 troops from nearly 8,000. Representatives of foreign governments and international bodies meeting in Madrid this week said they noted the AU's desire to strengthen the force.
"Meetings in New York and Madrid about Somalia sponsored by UN and the Somali Contact Group are against the actual desire of the Somali people," Aweys told Reuters by telephone.
"We believe Somalia belongs to Somalis and they have a right to decide their destiny. So we condemn the outcome of the conferences particularly the support of AMISOM and deployment of further AU troops because AMISOM's presence will not bring any solution to Somalia's situation."
The presence of the AU troops in Mogadishu has angered the rebels and some Somalis, who see the deployment as interference.
Soldiers from Burundi and Uganda guard the port and airport and shield President Sheikh Sharif Ahmed from attack but are often the target of rebel strikes.
Al Shabaab claimed responsibility for bomb attacks in Uganda in July that killed over 70 people, which it said were in retaliation for the country's deployment in Somalia.
"AU always talks of 75 people killed in the Ugandan explosions but never the dozens of Somali people dying almost daily in shelling from their AMISOM forces," Aweys said.
"We shall not lay down weapons and our struggle will not stop because this will make it easy for the colonial powers to carry out their wishes."
