ABUJA (Reuters) - The main suspect behind twin car bombings in Nigeria's capital Abuja last month was also responsible for bomb attacks in the southern oil city of Warri in March, the secret service said on Wednesday.

The State Security Service (SSS) said Henry Okah, who is facing conspiracy and terrorism charges in South Africa over the Abuja attacks on October 1, travelled to Warri and wired the car bombs which were detonated on March 15 outside government talks about an amnesty programme.

Okah is one of the suspected leaders of the Movement for the Emancipation of the Niger Delta (MEND), a militant group which this week claimed the kidnapping of seven French, U.S., Indonesian and Canadian workers from an oil rig off Nigeria's southern coast.

"It has been uncovered that Henry Okah came from South Africa for the Warri bombings, purchased the cars which were then moved to the welder and later to the house of one of the suspects ... where he personally wired the bombs," SSS spokeswoman Marilyn Ogar told a news conference.

"Subsequently, on March 14, Okah departed the country, apparently to create an alibi for himself over the bombings that took place the next day," she said.

The first vehicle exploded on an expressway several hundred metres from the Delta state governor's compound in Warri, the second at the gates of the building, in attacks which security sources later said appeared to have been a dry run for October 1.

Both the Abuja and the Warri bombings were claimed by MEND.

Security sources say Okah was long one of the masterminds behind the group, helping to supply weapons and to organise years of attacks on infrastructure including pipelines and flowstations in Africa's biggest oil and gas industry.

Okah, who lives in South Africa and has denied being MEND's leader, has also denied any links to the October 1 blasts. His lawyers say prosecutors have yet to produce any evidence linking him to the explosions.

THREAT OF FURTHER ATTACKS

The claims of responsibility by MEND were an embarrassment for President Goodluck Jonathan, the first Nigerian head of state to come from the Niger Delta, who brokered an amnesty with militants in the region last year.

Jonathan faces elections next April and his comments that the Abuja bombs had nothing to do with the Niger Delta and that MEND's name was used as a cover have already risked becoming a campaign issue, leading rivals to accuse him of partisanship.

Further unrest in the Niger Delta is the last thing he needs as the election season gets underway.

Former MEND commanders who laid down their weapons as part of last year's amnesty programme have condemned the October 1 bombs, which killed at least 10 people, and the secret service has said it has arrested the main culprits.

But the group has always been highly factionalised and some of its fighters remain active.

MEND claimed responsibility for an attack on an offshore oil rig operated by exploration firm Afren late on Sunday in which two Americans, two Frenchmen, two Indonesians and a Canadian were seized.

It also says it is holding three more Frenchmen and a Thai national kidnapped from an oil services vessel in September and has threatened more attacks on the energy industry.

Police in Bayelsa, one of the three main oil-producing states in the Niger Delta, said on Wednesday they had arrested two men who were trying to rig a car with explosives at a bus station in the state capital Yenagoa.

They were seized after part of the home-made device went off, severing one of the man's hands, Bayelsa state police commissioner Aliyu Musa said.

Police said the two men admitted having carried out previous attacks on behalf of MEND.