(Adds rebel spokesman's comments)

By Austin Ekeinde

PORT HARCOURT, Nigeria, March 23 (Reuters) - Gunmen kidnapped three foreign workers in two separate armed attacks in Nigeria's oil producing delta on Friday, authorities said.

Expatriate abductions have become an almost weekly occurrence this year in the world's eighth largest oil exporter, and thousands have fled the Niger Delta since violence surged last year.

Attackers in three speed boats stormed the construction yard of German building contractor Bilfinger Berger <GBFG.DE> in the region's largest city Port Harcourt at dawn and abducted the company's Dutch security manager after a three-hour gunfight, security sources said.

Hours later in the nearby city of Warri, gunmen took at least two foreign staff of Nigerian construction firm Setraco from their workplace. The hostages included a Lebanese national and possibly an Indian, but details were still sketchy, a company source said.

After dozens of foreigners were abducted in the first two months of this year, there had been a lull this month and the last two remaining foreign hostages -- two Italians working for ENI <ENI.MI> unit Agip -- were released unharmed on March 15.

REBELS BLAME RANSOM-SEEKERS

The Movement for the Emancipation of the Niger Delta (MEND), which held the Italians for more than three months, last week threatened to take more hostages. But MEND's spokesman denied involvement in the latest abductions and blamed ransom-seekers.

Army spokesman Sagir Musa said he did not know where the kidnappers took the Dutch man, but said the local community was helping troops with their search.

Militant attacks, invasions and kidnappings have forced oil companies to reduce Nigerian output by about 20 percent since February 2006.

MEND says it is fighting for regional autonomy in the oil producing delta and other political ends, but the line between militancy and crime is blurred.

Landmark elections in Nigeria next month have also increased instability in the delta as the prospect of power changing hands has rekindled long-standing communal conflicts.

The vast region of mangrove-lined creeks, with impoverished fishing villages nearby multi-billion dollar oil facilities, is awash with small arms.

Poverty and lack of basic public services due to government corruption lie behind much of the violence in the delta, which accounts for all the crude oil produced in Nigeria.

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