* Rebels prevent some civilians from fleeing, residents say
* Kenyan military warns Somalis to stay away from rebel bases
* Eritrea denies it supplying arms to al Shabaab
* Incursions risk galvanising support for militants (.)
By Abdi Sheikh and Sahra Abdi
MOGADISHU, Nov 2 (Reuters) - Somalia braced for Kenyan air attacks on Wednesday and Islamist militants stopped civilians from fleeing at least one likely strike zone.
Kenya, which sent troops into lawless Somalia nearly three weeks ago to crush the al Shabaab militant network, said on Tuesday it planned "imminent" air raids on militant bases and warned residents to stay clear of them.
Kenya's warning of air bombardments was prompted by reports the al Qaeda-linked militants had received two consignments of weapons, flown into the rebel-controlled town of Baidoa.
Eritrea dismissed media reports it had delivered the arms cache as "outright lies" meant to dirty its reputation.
Some 24 hours after Kenya gave its warning, there were no raids reported in the ten rebel strongholds where it had advised civilians to stay clear of insurgent bases.
"(Al Shabaab) ordered us to stay and die at the hands of Christian Kenya, to dwell in paradise," Abdikadir Weydow, a resident of the southern town of Afmadow, told Reuters.
Afmadow, a rebel bastion and strategic transit point for contraband smuggled through rebel-controlled Kismayu port, is seen as a likely flashpoint for a confrontation between Kenyan forces and al Shabaab militants.
Kenyan and Somali government troops, as well as militia nominally allied to Somalia's government, have set up forward positions close to Afmadow.
In many other towns, including Baardheere, Baidoa and Afgoye, many people were preparing to escape, hoping to lie low in the bush or reach the Kenyan frontier.
"We are determined to flee to the jungle. We cannot stay in a town which is to be bombed," said Baardheere resident Yusuf Guled.
Others, too poor to afford the transport or encumbered by elderly relatives, were hunkering down in anticipation of an aerial assault.
Kenya's army spokesman Emmanuel Chirchir told Reuters on Tuesday its forces would not target civilians, but warned they needed to stay away from rebel bases to avoid being caught up in the fighting.
Kenya was tracking two consignments of weapons destined for al Shabaab that had been flown into the Horn of Africa country in the past two days, and would strike any rebel bases where the arms were delivered, he said.
Eritrea denied it had sent the arms cache.
"The government of Eritrea states categorically that these accusations are pure fabrications and outright lies as Eritrea has not sent any arms to Somalia," Eritrea's foreign ministry said in a rare statement.
"Tuesday's baseless accusations are the latest product of a misinformation campaign orchestrated to undermine Eritrea and frustrate its constructive regional and international engagement, it said. Eritrea rarely comments on its policies.
Kenya sent ground troop reinforcements towards the Somali border on Wednesday, a Reuters witness said.
Columns of armoured vehicles and military trucks were seen heading to two frontier points from the town of Garissa, in northern Kenya, late on Tuesday and early Wednesday.
A military source said one convoy would cross the porous border at Liboi while the second would enter through the Hulugo border point and join the pro-Somali government Ras Kamboni militia group.
A Kenyan military officer recently arrived in Garissa from inside Somalia told Reuters preparations were being completed for a major offensive against the insurgents.
"We are set for the mother of all the battles. It's no longer jet fighters alone that will fight them, we are all moving to hit al Shabaab," said the officer who declined to be named.
Kenya, east Africa's biggest economy, has long looked warily at its lawless neighbour and accuses al Shabaab of frequent attacks on its security forces inside Kenya.
Residents in the Kenyan border town of El Wak said suspected Somali gunmen fired indiscriminately at buildings and a mortar hit a guesthouse in the early hours of Wednesday.
"There were gunshots from the Somali side last night. A mortar also landed on our side. No one was killed or hurt in the incident," said North Eastern Province police commander Leo Nyongesa.
Kenya is the latest in a string of foreign powers to send in soldiers in a bid to stabilise the chaotic country.
Some Horn of Africa analysts says Kenya's deployment inside Somalia lacks the military muscle to deal a mortal blow to al Shabaab, blamed for a spate of kidnappings and attacks on security forces in Kenya.
Kenya's offensive, they say, risks galvanising support for the militants.
"We believe we shall die in Kenya's bombardment. Kenya's action will create animosity and trouble between Kenyan and Somali civilians," Afmadow resident Weydow said. (Additional reporting by Richard Lough, Sahra Abdi and Yara Bayoumy in Nairobi and Noor Ali in Garissa; Writing by Richard Lough; Editing by Louise Ireland)