- A dusty hot wind blew freely through Liboi's only street. The Muslim town on the border with Somalia had shut down for afternoon prayers. But in the backyard of one of the iron-and-wood cabins, activity was rife. Laden with heavy plastic bags, a group of weary women and children trickled through a narrow gate. Their minders led them to a makeshift shed and gave them ice-cold water. "Those have just crossed the border from Somalia into Kenya," said Abdi Yusuf, an aid worker in Liboi.

The refugees had walked for several hours on small sandy paths to avoid Kenya's police. They fled from hardline Islamist al Shabaab militia which controls their home town of Dobley in Somalia, Yusuf said. "A Somalia under al Shabaab is not a place for us to stay, so we found all means possible to come into Kenya," said Mohamed Nur, another Somali refugee headed to a refugee camp near the town of Dadaab with his young son.

"Al Shabaab is witch-hunting all those who don't support them," he added. Kenya closed its 680-km (420-mile) border with Somalia in 2007. But hundreds of Somalis sneak in illegally every day, fleeing violence in their homeland that has killed some 21,000 civilians since the start of 2007 and uprooted 1.5 million people.

Human smugglers, middlemen, hotel and bus owners collude with corrupt police officers to move Somali refugees into and around Kenya in a lucrative trade. MORE REFUGEES The U.N. refugee agency UNHCR said 2,730 refugees had entered Kenya from Somalia in January - lower than last year's average of 5,000 a month. But that number could be about to go up again with the start of a long-promised offensive by Somali government forces against the al Shabaab rebels and other Islamist insurgent groups. In anticipation, Kenya has boosted security on the border this year. But controlling the influx of refugees across the desert frontier remains difficult, Kenyan authorities say.

"The border is too long for us to have forces at each point but we try our level best to carry out patrols to make sure we stop people from coming in," said Winston Murungi, the commissioner of Kenya's border district of Lagdera. Corruption within security forces further slackens controls.

"There are individuals who take money from illegal aliens moving from Liboi and other borders to Nairobi," said James Ole Seriani, police chief for Kenya's northeastern region. According to different accounts, a refugee can pay a bribe of $50 to $100 at each of the several checkpoints on a highway between the border towns of Liboi and Mandera and Nairobi.

That compares to an average monthly salary of $250 for police officers in Kenya. "We are doing our level best to catch the corrupt officers and stop this illegal business," Seriani said by telephone from Garissa. But ending human smuggling is "a major challenge because it involves hundreds of dollars changing hands", he added.