* Over 200 girls abducted from secondary school in April
* Military says force to rescue girls too risky
* Senate president rules out prisoner swap, negotiation
* Ambush kills four soldiers in Plateau state (Adds details, background, Jos attack, byline)
By Tim Cocks
LAGOS, May 26 (Reuters) - Nigeria's military knows where the more than 200 girls abducted by Boko Haram are but has ruled out using force to rescue them, the state news agency quoted Chief of Defence Staff Air Marshal Alex Badeh as saying on Monday.
Seven weeks since Boko Haram militants abducted more than 200 girls taking exams at secondary school in the remote northeastern village of Chibok, little is known of their whereabouts or what the military is doing to get them out.
"The good news for the parents of the girls is that we know where they are, but we cannot tell you," Badeh was quoted by the News Agency of Nigeria (NAN) as saying.
"But where they are held, can we go there with force? We can't kill our girls in the name of trying to get them back."
Most officials think any raid to rescue them would be fraught with danger and probably not worth the risk that the girls would be killed by their captors - an Islamist group that has shown a high degree of ruthlessness in killing civilians.
Since the girls were captured, according to a Reuters count, at least 470 civilians have died violent deaths in various locations at the hands of Boko Haram, which says it is fighting to establish an Islamic state in religiously mixed Nigeria.
Nigerian President Goodluck Jonathan has branded the group an "al Qaeda of West Africa".
Britain's BBC reported on Monday that a deal was close to being agreed to rescue the girls in exchange for Boko Haram prisoners - a demand the group had made public - but that it was called off at the last minute.
Over the weekend, Senate President David Mark, the country's number three, ruled out a deal with Boko Haram, whose name means "Western education is a sin" in the northern Hausa language.
"This government cannot negotiate with criminals and ... will not exchange people for criminals. A criminal will be treated like a criminal," he was quoted by local media as saying.
Nigeria accepted help from the United States, Britain, France and China last week and around 80 U.S. troops have started arriving in neighbouring Chad to start a mission to try to free the girls. Surveillance drones are scanning the Sambisa forest, where parents say the girls were last sighted.
But the forest covers 60,000 square km (23,000 square miles), more than twice the size of Rwanda, and the rebels know the terrain intimately.
Nigeria and its neighbours say Boko Haram, which has killed thousands during its five-year insurgency in Africa's top oil producer and largest economy, now threatens the security of the whole region.
Gunmen killed four Nigerian soldiers on Monday in an ambush on a military patrol in central Plateau state, about 180 km (110 miles) southeast of Jos, a local government official said, although it was not confirmed if this was Boko Haram.
The Islamists have made inroads into Plateau state in the past month, setting off a bomb in Jos last Tuesday that killed 118 people, although it failed to trigger the kind of widespread communal violence to which Plateau has been prone for a decade.
Over the years Boko Haram has slowly morphed from a radical clerical movement opposed to Western cultural influences to a violent network of Islamist fighters and associated criminal gangs that kill, kidnap and extort money.
The insurgents initially attacked security forces and government officials after they launched their uprising in the northeast city of Maiduguri in 2009. When Jonathan ordered an offensive a year ago to flush them out, civilians formed vigilante groups to help out, which then made them targets too. (Additional reporting by Buhari Bello in Jos and Lanre Ola in Maiduguri; Editing by Alison Williams)