Mogadishu, Somalia (CNN) -- Somalia's top militant group may soon counter the government's recent thrust against them with an offensive of their own.
African Union intelligence sources say Al-Shabaab is massing hundreds of troops in the African nation's embattled capital of Mogadishu for a show of force during the Muslim month of Ramadan, which begins Monday.
The sources said Al-Shabaab received a shipment of weapons by sea in preparation for this offensive. They believe that Al-Shabaab allies in Yemen shipped the items across the Red Sea to Somalia.
Ramadan is a period of fasting and religious reflection. Islamic radicals have chosen the period to carry out attacks against their enemies during the holy month.
Fighting has intensified this summer as Somali government forces -- backed by troops from the African Union -- have battled insurgents from the militant Al-Shabaab.
Somali government forces on Thursday launched an offensive on Islamist strongholds in northern Mogadishu.
The U.N. refugee agency on Friday expressed concerns about "the protection of civilians" in Mogadishu amid the fighting.
But African Union forces said the offensive was launched to counter Al-Shabaab attacks on displaced persons camps.
"An offensive on Thursday by pro-government forces in and around the important Bakara and Balcad markets has increased the risk for Mogadishu's citizens as well as the estimated 100,000 internally displaced people (IDP) who have fled drought and famine in neighbouring regions in recent months," the U.N. High Commissioner for Refugees said.
Since September, African Union forces said they have steadily been taking territory in the capital from Al-Shabaab.
The fighting comes also as Somalia is battling its worst drought in 60 years and tens of thousands of famine-stricken people are walking for days into both Kenya and Ethiopia in search of food and water and away from the conflict between government forces and rebels.
The United Nations on Friday said it is asking for a further $1.4 billion to help starving people across the Horn of Africa. The request lifts the Horn of Africa appeal to $2.4 billion, of which $1 billion has been received so far.
"More than 12 million people -- in Kenya, Ethiopia, Somalia and Djibouti -- are in dire need of help, and the situation is getting worse," said Valerie Amos, U.N. Emergency Relief Coordinator and head of the Office for the Coordination of Humanitarian Affairs.
"If we are to avoid this crisis becoming an even bigger catastrophe, we must act now."
Meanwhile, Erastus Mwencha, the African Union Commission deputy chief, visited Mogadishu to assess the humanitarian situation and announced an international pledging conference August 9 in Ethiopia.
The African Union has already donated $500,000 of assistance in the famine.