* Britain offers more help in hunt for girls
* At least four soldiers killed in Boko Haram ambush
* France security summit to discuss Boko Haram (Adds quotes, details on ambush)
By Isaac Abrak
MAIDUGURI, Nigeria, May 14 (Reuters) - Nigeria's president has rejected an offer from Islamist rebel group Boko Haram to exchange schoolgirls it abducted for imprisoned militants, but the government is open to broader talks with the rebels, a visiting British minister said.
President Goodluck Jonathan is under pressure to crush the rebels who have killed thousands in their campaign for an Islamist state and to free the girls whose abduction a month ago has sparked global outrage.
Government officials initially said they were exploring all options with respect to the swap proposal and later said they were willing to negotiate with Boko Haram without specifying whether any putative talks might include an exchange for the girls.
Jonathan further refined that position on Wednesday during talks with Britain's Minister for Africa Mark Simmonds.
"He (Jonathan) made it very clear that there would be no negotiations with Boko Haram that involved a swap of abducted schoolgirls for prisoners," Simmonds told reporters after meeting Jonathan.
"What he also made very clear to me was that he wanted his government to continue a dialogue to make sure a solution could be found and that security and stability could return to northern Nigeria for the medium and the long term," he added.
Rebels stormed a school in the northeastern village of Chibok a month ago and seized 276 girls who were taking exams. Some have escaped, but about 200 remain missing. On Monday, Boko Haram leader Abubakar Shekau released a video showing some of the girls in captivity and offered the swap.
The abductions have triggered a social media campaign under the Twitter hashtag #BringBackOurGirls and the United States, Britain, France and Israel have offered help or sent experts to Nigeria to assist the effort.
Britain has offered a Sentinel surveillance plane, a team embedded at Nigerian military headquarters, a team embedded with international groups analysing intelligence from the hunt and more funding for a Safe Schools Initiative, Simmonds said.
"The offer has been accepted by President Jonathan ... so we are working on the detail of that and how we might facilitate a quick implementation," Simmonds said.
Boko Haram killed at least four soldiers in an ambush on Tuesday near the village where the girls were abducted. The clash highlights the obstacles facing an international hunt for the girls that is gathering pace as well as the willingness of the rebels to confront the Nigerian army.
The soldiers were killed in a night ambush outside the northeastern city of Maiduguri and several insurgents also died in the firefight, according to a statement on Wednesday from Defence Headquarters.
"Troops on patrol around Chibok were ambushed by insurgents yesterday. Troops engaged the insurgents in a fierce combat and extricated themselves from the ambush killing several insurgents. Four soldiers, however, lost their lives during the ambush," said a statement from Defence Headquarters.
"On evacuation of the remains of the fallen troops, the General Officer Commanding addressed the troops who registered their anger about the incident by firing into the air. The situation has since been brought under control," it said.
Soldiers interviewed by Reuters said the ambush happened after a commanding officer instructed his unit to return to Maiduguri in the evening, something the troops were unwilling to do on the grounds it might expose them to attack.
In interviews, they gave differing numbers of those killed, but all exceeded the four stated by Defence Headquarters. One account said six died while another said 12 had been killed.
Publicity over the abductions has cast a harsh spotlight on Nigerian military efforts to secure the girls' release and quash the insurgency and also opened an avenue for diplomatic and security cooperation.
France is due to hold a security summit in Paris on Saturday with Jonathan, leaders from neighbours Benin, Cameroon, Chad and Niger and other African countries, as well as representatives from the United States and Britain.
Jonathan said he welcomed the summit, which will discuss how to intensify collaboration against Boko Haram and other radical groups.
"If we all collaborate more, it will easier to eradicate Boko Haram and terrorism," he said in a statement.
Defence chiefs from the Economic Community of West African States also met in the Ghanaian capital Accra on Wednesday to discuss regional action on Boko Haram. (Additional reporting by Lanre Ola in Maiduguri, Kwasi Kpodo in Accra and Felix Onuah in Abuja, writing by Matthew Mpoke Bigg; editing by Bate Felix and G Crosse)