* Government promises to track down culprits
* Uncertainty about who is behind attacks worries locals
* Hundreds of families flee Lamu region to find safety
* Al Shabaab has claimed responsibility for other attacks (Adds al Shabaab denies sent leaflets, government announces arrests)
By Joseph Akwiri
MOMBASA, Kenya, July 8 (Reuters) - Armed assailants have attacked a camp of police reservists and burned down nearby homes and businesses, a regional official said on Tuesday, as violence escalates on Kenya's northern coast. But there were no reported deaths this time.
In the nearby coastal town of Lamu, residents also reported finding leaflets strewn about threatening Christians and the government. The leaflets purported to be from the Somalia's al Shabaab Islamists, but the group denied it sent them.
The government has promised to track down those behind attacks on coastal towns and villages, most of which have been deadly, but worried locals say the repeated assaults show the state is not doing enough or is powerless to stop them.
Hundreds of families have fled homes in Lamu County, where the worst attacks have occurred, sheltering in camps near police posts or fleeing the region altogether.
A once vibrant tourist trade in the area and all along the coast has been hammered.
"The assailants attacked a camp belonging to Kenya police reservists in Hindi and burnt it down before they burnt down other houses and businesses in Amu Ranch," Lamu County Commissioner Miiri Njenga told Reuters.
The attackers, who struck late on Monday, also set vehicles on fire.
Hindi was the site of an attack on Saturday that killed 9 people and Amu Ranch is nearby. Both are close to Mpeketoni area, the site of two assaults in mid-June that left 65 dead.
Raising tensions, residents of Lamu, a historic Arab trading port and capital of the county, said leaflets were spread in the town warning Christians and the government not to victimise Muslims, without saying what could happen if they did.
The leaflets, which were pinned to electricity polls and trees or simply found strewn on the ground, bore the emblem of the al Shabaab Islamist group, which claims most attacks and has vowed to drive Kenyan and African Union forces out of Somalia.
But the Somali group denied using such techniques. "We use something better: the power of force," Sheikh Abdiasis Abu Musab, al Shabaab's military operations spokesman, told Reuters.
The coast, notably around Lamu, has a history of land disputes where traditional coastal peoples accuse ethnic groups from up country of taking their property. Among relative newcomers are Kikuyus, President Uhuru Kenyatta's ethnic group.
"We think they (the leaflets) are meant to instil fear in locals who are not originally from here (Lamu) and we are trying to establish the source and authors," Njenga said.
The government has said recent attacks had nothing to do with al Shabaab and instead blamed local groups - seen as directed at their political opponents. Police have also suggested a coastal separatist group could have a role.
Deputy President William Ruto, who visited the Lamu area this week and gave police 48 hours to catch those behind the recent attacks, said on Twitter that 50 suspects, including one with gunshot wounds, had been arrested in Lamu County and Tana River area, where there was an assault at the weekend.
The uncertainty about who is to blame has made many Kenyans even more nervous, because they fear that the security forces will struggle to prevent more attacks if officials cannot agree on who is behind them.
One former security official suggested the attacks could involve several sets of people. Al Shabaab might be using the knowledge of local operatives with grievances to stoke ethnic rivalries, he said.
He also said the Mombasa Republic Council, an outlawed coastal separatist group, could be taking part. The group has consistently denied any role in the killings, but some analysts say it might be regrouping and splinter groups may have formed. (Additional reporting by Feisal Omar in Mogadishu; Writing by Edmund Blair; Editing by Louise Ireland and Drazen Jorgic)