LAGOS, June 2 (Reuters) - Gunmen dressed as riot police have abducted some foreign workers from the residential compound of oil services giant Schlumberger <SLB.N> in Nigeria's oil city Port Harcourt, police said on Saturday.

There has been an upsurge in violence against foreign workers in the world's eighth largest oil exporter since last year, and about 30 expatriates are currently being held by various armed groups in the anarchic Niger Delta region in southern Nigeria.

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