The AU Mission in Somalia's plan for the final assault on the key Somali port city of Kismayu could be hauled back to the drawing board after three of four Ugandan attack helicopters headed to shore up the meticulously planned operation were involved in a serious mishap over Kenyan airspace.

Two of the three Russian-made Mi-24 attack helicopters were still missing by Monday evening, while the other crash-landed in the thick forest slopes of Mount Kenya in central Kenya.

Uganda military spokesperson Col. Felix Kulayigye told reporters in Kampala that the fourth –a Mi-17 transport helicopter with 13 crew-- had made it to Mogadishu.

Military sources said Tuesday that the three Mi-24 choppers were badly damaged and would most likely be written off.

Military sources say the mishap in which the helicopters, bought expensively by Uganda in 2011, would significantly affect the plan to attack Kismayu as the Ugandan air wing was to lead planned airstrikes.

The Kenyan contingent in the mission was charged with naval operations while both countries were moving ground troops to squeeze the Al-Qaeda-allied militants.

Kismayu is reported to be the financial and logistical hub for the terror group and is their last remaining stronghold after a series of territorial losses on the battleground.

Sources said that the assault on Kismayu, which started over the weekend with aerial surveillance and reconnaissance flights conducted by Unmanned Aerial Vehicles (UAVs), or drones, could be postponed.

The commanders were yesterday re-called back to Nairobi for a series of crisis meetings expected to start at the weekend.

Amisom spokesperson Eloi Yao Monday said he was not in position to comment about the mishap and how it would affect the plan for Kismayu. He declined further comment.

The AU forces, fighting alongside Somali government troops, were expected to start a major assault on Kismayu, the remaining stronghold of the Al-Shabaab by the end of this week.

The Ugandan and Burundian contingents were expected to support Kenyan ground troops with four infantry battalions.

Some of these troops are already on their way to Kismayu, having taken control of Afgoye and a string of coastal villages in Lower Juba region to the south.

Other troops were to be inserted into battle by air using the Mi-17 transport choppers.

The seemingly sophisticated and well-choreographed military strategy designed was agreed after top commanders of the reshaped African Union Mission in Somalia (Amisom) concluded their planning meetings on August 9 in the Kenyan capital of Nairobi.

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