(Recasts with arrest of Boko Haram member, updates death toll, changes dateline)
By Anamesere Igboeroteonwo and Joe Hemba
ONITSHA/DAMATURU, Nigeria, June 18 (Reuters) - Nigerian security forces discovered a senior member of the militant Islamist group Boko Haram among a convoy of nearly 500 travellers arrested this week in the southern state of Abia, a military spokesman said on Wednesday.
News of the arrest comes as militants are killing civilians almost daily, particularly in the largely Muslim north. At least 14 people were killed on Tuesday when a bomb tore through a venue in the northeast where fans had gathered to watch a World Cup match.
The presence of a senior Boko Haram member in Nigeria's south would stoke fears Islamist militants are pushing into regions well beyond their northern stronghold - including in the mainly Christian south.
It would also raise concern the group, which drew worldwide attention in April when it kidnapped more than 200 girls from a school in northeast Nigeria, could eventually attempt attacks in the oil-rich Niger delta.
Officials said on Monday that nearly 500 people believed to be northerners had been arrested overnight while travelling through the south in convoy of more than 30 vans.
"The army is screening the 486 persons being held in Abia state," defence spokesman General Chris Olukolade told Reuters on Wednesday. It had identified a "kingpin belonging to Boko Haram" among them, he said.
The army did not release the suspect's name or photograph.
Some of those arrested had said they were headed for southern city of Port Harcourt to look for work, the government has said.
Further stoking fears that militants are targeting the south, police in neighbouring Imo state defused three bombs found at a Christian church over the weekend, a local police spokesman said. Six people have been arrested, he said.
Boko Haram has shown its growing reach in recent months by moving beyond its heartland in Nigeria's weakly governed northeast. At least 118 people were killed in devastating back-to-back bomb blasts in the central city of Jos last month.
WORLD CUP BLAST
On Tuesday, at least 14 people, including young children were killed, when a bomb targeted a venue in the northeast town of Damaturu where fans had gathered to watch the World Cup on television.
Some people at the scene told Reuters an attacker dropped a device in front of the venue, while others said it was the work of a suicide bomber.
No one claimed responsibility for the blast, but Damaturu and the surrounding Yobe state are at the heart of the five-year-old insurgency by Boko Haram.
The group was blamed for a an attack on another venue screening soccer matches in the northeastern state of Adamawa that killed at least 14 people and wounded 12..
A Reuters reporter at Damaturu's General Sani Abacha Specialist Hospital counted over a dozen people dead - including small children - and at least 20 wounded.
Police on Wednesday confirmed that the death toll had risen to 14.
The Nigerian government has advised people to avoid gathering in public to watch the World Cup, concerned about potential attacks.
Many fans in soccer-mad Africa rely on informal venues - often open-sided structures with televisions set up in shops and side streets - to watch live coverage of the sport.
Boko Haram - whose name roughly translates as "Western education is sinful" - has declared war on all signs of what it sees as corrupting Western influence.
The group has killed thousands since 2009 in its push to create an Islamic state in religiously mixed Nigeria. (Reporting by Joe Hemba; Writing by David Dolan; Editing by Larry King)