* Forces find 10-year-old girl strapped with explosives
* President Jonathan pledges $500 mln for victims
* Militants blow up two mosques in northeast's Potiskum (Adds confirmation from government, quotes, Jonathan statement, witness)
By Nneluke and Ikemfuna
KANO, Nigeria, July 30 (Reuters) - A suicide bomber blew herself up at a college in northern Nigeria's biggest city of Kano on Wednesday, killing six people in the fourth such attack there in less than a week, the government said.
Six other people were critically wounded by the bomber who targeted youths looking at a notice board for national youth service in Kano Polytechnic, government spokesman Mike Omeri said.
There was no immediate claim of responsibility, although militant group Boko Haram, which is fighting for an Islamic state in religiously-mixed Nigeria, has repeatedly bombed Kano as it radiates attacks outwards from its northeast heartlands.
Omeri added that security forces had arrested three Boko Haram suspects in Katsina state, two of them female, on Tuesday. One was a 10-year-old girl who had had an explosive belt strapped to her by the others, he said.
Using female suicide bombers in the city appears to be a new tactic of Boko Haram, although they have used them on occasion for years in the northeast.
Two female suicide bombers blew themselves up at a trade show and a petrol station in Kano on Monday, killing one other person and injuring at least six others.
On Sunday, a female suicide bomber killed herself but no one else while trying to target police officers.
Wednesday's attack was "at the Kano State polytechnic where some students had gone for the collection of their call-up letters for the National Youth Service Corps," Omeri said in a statement.
"We wish to assure Nigerians that the government is putting all efforts and resources into ... countering the violent insurgency by Boko Haram," he said, after giving the death toll.
In a separate incident on Tuesday, two suicide bombers killed 13 people in attacks on two mosques in the town of Potiskum, in Yobe state in the northeast, medical official Bala Afuwa, who received the bodies at a local hospital, told Reuters by telephone on Wednesday.
"Two of my uncles were killed," said resident Mohammed Abubakar, whose family home is next to one of the mosques that were attacked. "They had just returned from the mosque."
President Goodluck Jonathan, who has come under heavy criticism for failing to end the five-year-old rebellion, pledged $500 million on Wednesday towards Nigerians living in states that are worst affected by Boko Haram violence.
Though much of the violence is concentrated in the remote northeast, the group has have struck across Nigeria in several attacks since April. On Sunday, they mounted a cross-border attack into Cameroon, killing at least three people there and kidnapping the wife of the vice prime minister. (Reporting by Nneluke Ikemfuna; Additional reporting by Joe Hemba in Damaturu, Tim Cocks in Lagos and Camillus Eboh in Abuja; Writing by Tim Cocks; Editing by Angus MacSwan/Ruth Pitchford)