The African Union special envoy for Somalia on Wednesday urged countries waging war on al Shabaab to keep an open door to negotiations with militants who are willing to lay down their weapons.

Neighbouring Kenya sent hundreds of troops into southern Somalia in mid-October to crush the Islamist insurgents it blames for a series of kidnappings on its soil and regular cross-border attacks. Its air force has launched sporadic strikes on what it says are rebel targets.

Ethiopia also sent dozens of military trucks and armoured vehicles into central Somalia over the weekend, witnesses said.

I'm not saying that Kenya should consider negotiating with al Shabaab. I'm saying that all of us engaged in the effort to bring stability to Somalia should not shut the door to combatant elements who would want to put aside their weapons and talk politics, said Jerry Rawlings, former president of Ghana and now AU representative for Somalia.

Even as we are doing combat ... let's also create an opening for them to come on board politically. Let's not shut all the doors to them, Rawlings told a news conference in Nairobi.

The AU has backed Kenya's pursuit of al Qaeda-linked al Shabaab inside Somalia to secure stability in the Horn of Africa country and ensure Kenya's own security. But Rawlings warned on Wednesday that the conflict would be complex and bloody.

It is important to maintain the support of the local populace, he said. The exercise that your armed forces are going to undertake is a complex one, because a lot of it will involve fighting in urban dwelling areas, and you will have to be prepared for some casualties.

Somalia's Prime Minister Abdiweli Mohamed Ali told Reuters in September that his government was open to talks with al Shabaab's top commanders and informal talks had been held.

Al Shabaab, fighting to impose a harsh interpretation of sharia law on the nation, have vowed to revenge against the region's biggest economy and bring the flames of war to its neighbour.

LIBERATION?

Kenya's Defence Minister Yusuf Haji, who attended the news conference, declined to say whether the east African nation would negotiate with any al Shabaab rebels. He reiterated that Kenya had the right to pursue the enemy inside Somali territory.

What we are trying to do is liberate as many areas of Somalia as possible, Haji said.

He urged Rawlings to help secure the assistance of the AU peacekeeping force, AMISOM, to come and take position in the liberated areas whenever it is necessary.

About 9,000 Ugandan and Burundian soldiers make up AMISOM, which is propping up the shaky Western-backed Somali government and now controls much of the capital Mogadishu after al Shabaab withdrew in August from the coastal city.

The Horn of Africa country has been racked by violence since the overthrow of dictator Siad Barre in 1991 allowed first warlords, then Islamist militants, to step into the political vacuum.

Ethiopia publicly denies its forces are inside Somalia. Addis Ababa has said a decision on whether to join the assault against al Shabaab in some form would be taken on Friday at a meeting of east African heads of state.

Ethiopia entered Somalia in December 2006, with tacit U.S. backing and at the invitation of a Somali government, and left under a cloud in 2009 after routing a previous Islamist administration. But Ethiopia's perceived occupation deeply angered many Somalis, helping to swell the ranks of al Shabaab.

Asked if the Ethiopians' re-entry into Somalia would be well received, Rawlings said: Quite frankly I think they have learnt the necessary lessons and the feedback we are getting is that they are coming on board, we are on the same page.

Al-Qaeda Ghana Ethiopia Kenya