* Bombs targeted crowded business, market area
* Islamists Boko Haram a prime suspect, insurgency spreads
* President Jonathan pledges bigger force to fight Islamists (Recasts with higher death toll, adds quotes)
By Adamu Jonah and Anamesere Igboeroteonwu
JOS, May 20 (Reuters) - Back-to-back bomb blasts killed at least 46 people and wounded 45 in the crowded business district of the central Nigerian city of Jos on Tuesday, police said, in an attack that appeared to bear the hallmarks of Boko Haram insurgents.
There was no immediate claim of responsibility, but the militant Boko Haram, which has set off bombs across the north and centre of Nigeria in an increasingly bloody campaign for an Islamic state, was likely to be the prime suspect.
Boko Haram grabbed world headlines by abducting more than 200 schoolgirls over a month ago from the northeast village of Chibok. Britain, the United States and France have pledged to help rescue them.
If the Jos attack was the handiwork of Boko Haram, it would show their growing reach in Africa's top oil producing and most populous country, striking out beyond their heartlands in Nigeria's semi-arid and weakly governed northeast. Several bombs have exploded outside that region over the past month.
A Reuters reporter saw 10 bodies burned beyond recognition at the bomb site opposite a hospital at Terminus, the downtown area of Jos housing shops, some offices and a market. Plateau state Police Commissioner Chris Olakpe confirmed the toll of 46, adding that several wounded had been taken to hospital.
"The first explosive went off around 3 p.m. The second was about 3.30 while people gathered to help the victims," he said by telephone. "This is a very busy area of Jos metropolis." The back-to-back blast tactic, whose aim is to maximise civilian casualties, has also been used by militants in places like Iraq.
President Goodluck Jonathan condemned the blasts, calling the perpetrators "cruel and evil."
"The government remains fully committed to winning the war against terror, and this administration will not be cowed by the atrocities of enemies of human progress and civilization," he said in a statement emailed by his office.
He announced heightened measures to tackle the insurgents, including a multinational force around Lake Chad, comprising a battalion each from Chad, Niger, Cameroon and Nigeria.
Jos has been relatively free of attacks by Boko Haram, but it claimed responsibility for a bomb in a church in the highland city, as well as two other places, on Christmas Day in 2011.
The city is in the heart of Nigeria's volatile "Middle Belt", where its largely Christian south and mostly Muslim north meet, and surrounding Plateau state is often a flashpoint for violence, although the Christmas bomb failed to trigger any.
Tuesday's explosions burned several shops to the ground, shattering windows and spreading rubble in the road. Police sirens wailed as officers rushed to the scene.
"There was a loud bang that shook my whole house. Then smoke was rising," said Jos resident Veronica Samson. "There were bodies in the streets and people rushing injured to hospital in their cars."
For most of the past two years, the insurgency has been largely confined to Nigeria's remote northeast bordering Cameroon, Chad and Niger, where militants move easily across borders, but it appears once again to be spreading outwards.
A morning rush hour bomb killed at least 71 people at a bus station on the outskirts of the capital Abuja last month. Another in almost exactly the same place, in the suburb of Nyanya, killed at least 19 people at the beginning of May.
And a suicide car bomber killed five people on a street of bars and restaurants in the northern Nigerian city of Kano on Sunday evening, in an area mostly inhabited by southern Christians. (Additional reporting by Buhari Bello in Jos and Felix Onuah in Abuja; Writing by Tim Cocks; Editing by Mark Heinrich)