* Nigerian military offensive aims to crush Boko Haram
* Fighting has forced farmers to abandon land
ABUJA, July 1 (Reuters) - A Nigerian government rights watchdog said it had credible reports the country's own forces carried out extra-judicial killings, torture, rape and arbitrary detention in efforts to quell an Islamist insurgency in the northeast.
In an interim study compiled over June and seen by Reuters on Monday, Nigeria's National Human Rights Commission also said the violence had forced thousands of farmers to flee their land and warned the exodus could trigger a food crisis.
Nigeria's military did not immediately respond to a request for comment.
Amnesty International, Human Rights Watch and other campaign groups have made similar allegations over the past three years, but it was unusual to see them in a report compiled by a government organisation.
In May the military began its most concerted effort yet to end a four-year-old insurgency by Boko Haram, an Islamist sect that has killed thousands in a campaign to revive an ancient Islamic caliphate in Nigeria's northeast.
The commission's report said it had received credible "allegations of gross violations by officials of the JTF (Joint Task Force) ... summary executions, torture, arbitrary detention ... rape," without saying exactly where or when the atrocities took place.
The military says its offensive - which started when President Goodluck Jonathan declared a state of emergency in three states in the northeast - has driven Boko Haram fighters out of camps near the border with Chad, Niger and Cameroon.
But the rights commission said that since it began, "thousands have been forcibly displaced both within Nigeria and beyond; a farming season has been lost, threatening the region with a food security crisis."
"These consequences threaten a foreseeable humanitarian crisis on the region," it added.
A spate of attacks, including two on schools last month, showed the Islamists, who remain the biggest security threat to Africa's top oil producer, were still active.
The government rights commission said security had not been good enough to allow it access to the town of Baga, scene of fighting in April whose death toll is still disputed.
Local leaders said 185 people, mostly civilians, were killed while the military put the death count at 37, most of them insurgents.
The commission quoted a police report saying troops who had arrived to avenge the killing of a colleague "started shooting indiscriminately at anybody in sight, including domestic animals resulting in loss of lives and massive destruction of property".
Jonathan is scheduled to visit Borno state on Thursday to assess the security operation, presidency sources said on Monday. (Reporting by Camillus Eboh; Additional reporting by Felix Onuah in Abuja and Tim Cocks in Lagos; Writing by Tim Cocks)