By Abdoulaye Massalatchi and Joe Brock

NIAMEY/MAIDUGURI, Nigeria (Reuters) - Nigeria has asked its northern neighbor Niger for help in an offensive against Boko Haram insurgents, as West African states try to cooperate against a spread of Islamist violence.

An intelligence source in Niger said on Tuesday security had been tightened along the thinly populated border and military police were searching vehicles for Boko Haram fighters who might be fleeing the past week's onslaught on their bases in Nigeria.

A Nigerian minister delivered a request for assistance late on Monday in the Niger capital Niamey, but gave no details on what Niger's role may be. The success of al Qaeda associates in seizing a swathe of Mali last year prompted West African leaders to cooperate more against militants seeking an Islamic emirate.

Nigeria, oil-rich and Africa's most populous nation, worries that the four-year-old insurgency based in its remote northeast is being fed from abroad, through Niger, Chad and Cameroon. President Goodluck Jonathan declared a state of emergency in three states last week and launched a major military offensive.

Nurudeen Muhammed, a junior foreign minister, delivered the request for assistance from Jonathan to Niger's President Mahamadou Issoufou: "We currently have military operations under way in Nigeria in three federal states to combat terrorism and we would like to have Niger's support in the common fight against these terrorists," Muhammed told Niger state television.

Nigeria and Niger signed a mutual defense pact in October and soldiers from Niger, as well as Chad, joined Nigerian forces in an assault on Boko Haram fighters last month in the Nigerian town of Baga, on the semi-desert shores of Lake Chad.

The Niger intelligence source said 20 vehicles carrying suspected Boko Haram fighters had been spotted on Tuesday in Nigeria about 100 km (60 miles) from the border town of Bosso.

"The borders are huge, so even with joint efforts it will be difficult to prevent movement between countries, but we can stop as much as possible," a Nigerian military source said.

The fighting in Nigeria has pushed more than a thousand refugees across the porous 1,500-km (950-mile) frontier into Niger in the past few weeks, according to U.N. estimates.

REGIONAL THREAT

Nigeria claimed some early successes on Monday against Boko Haram, although military sources say the Nigerian troops have faced stiff resistance from seasoned rebels entrenched in the north and using cross-border routes to smuggle in weapons.

In a gesture to appease Islamist sentiment, the government said on Tuesday it would free several prisoners, including all women held on suspicion of Islamist violence.

In the northeastern city of Maiduguri, the heart of the insurgency, telephone lines were still down but some life returned to the town after a curfew was relaxed. Yellow rickshaws used as taxis returned to the sandy streets, which were still heavily guarded by military checkpoints and patrols.

Nigerian lawmakers from both houses passed motions on Tuesday approving emergency powers granted to the military.

Neighboring countries were alarmed last year when jihadist militants overran vast tracts of Mali's desert north, imposing a violent form of Islamic law and establishing training camps, some of which were attended by Boko Haram fighters.

France, the former colonial power in Mali, Niger, Chad and Cameroon, intervened in January to push the Malian rebels back and is now due to hand over in Mali to a U.N. peacekeeping force made up mostly of African troops, most of whom are Nigerian.

A spokesman for Nigeria's military denied reports it might have to pull some of its 1,200 troops out of Mali to support the campaign in the northeast against Boko Haram.

Many analysts say, however, that the domestic Islamist threat has distracted Nigeria from the leading role it has traditionally played in the west African region.

(Additional reporting by Tim Cocks in Lagos; Writing by Daniel Flynn and Tim Cocks; Editing by Alastair Macdonald)