* Lawmaker's death toll six times higher than government's
* Islamist groups pose increasing security threat
* Political challenge to President Goodluck Jonathan
MAIDUGURI, Nigeria, April 27 (Reuters) - A gunbattle between security forces and Islamist insurgents in Nigeria a week ago killed 228 people, a local senator said on Saturday, putting the death toll six times higher than the government's estimate.
A large number of civilian deaths will fuel accusations that the military acted heavy-handedly and failed to protect bystanders and might also increase pressure on the government to seek a negotiated settlement with the radical group Boko Haram.
There have been conflicting death tolls from the April 19 operation carried out by joint forces from Nigeria, Chad and Niger against Boko Haram insurgents in the remote northeastern town of Baga.
The Red Cross is investigating a report by locals that 187 people died, including many civilians, but defence spokesman Chris Olukolade said only 37 were killed.
The military barred aid agencies from entering Baga for several days after the attack. The Red Cross and some other agencies have since been given access but none has issued a definitive death toll.
"I personally visited three graveyards in Baga and counted 228 graves where victims had been buried," said Senator Maina Maaji Lawan, who represents the northern region of Borno state where the attack took place.
Lawan told reporters that 4,000 homes were also destroyed and without significant relief materials there could be a humanitarian disaster.
President Goodluck Jonathan has opened an investigation into the siege, including the actions taken by the military.
Boko Haram and other Islamist groups, such as the al Qaeda-affiliated Ansaru, have become the greatest threat to security in Africa's second largest economy and top oil producer.
Clashes between the military and Boko Haram are a near daily occurrence in the sect's northeast stronghold. Soldiers killed senior Boko Haram commander Mohammed Chad in an early morning raid on Friday, the military said.
President Jonathan is awaiting a report from a panel he set up to offer an amnesty to the insurgents if they give up their struggle for an Islamic state. Boko Haram has so far shown no interest in talks.
The sect, which wants an Islamic state in a country divided roughly equally between Christians and Muslims, has killed hundreds of people in guerrilla-style attacks. (Reporting by Ibrahim Mshelizza; Writing by Joe Brock; Editing by Robin Pomeroy)