UPDATE 4-Nigeria rebels attack oil facility, hijack tanker

* Militants say attack Chevron oil manifold

* Rebels also hijack chemical tanker, kidnap six

* Violence cuts at least 273,000 bpd of oil output

(Adds military spokesman, paragraphs 2-3, 9)

By Randy Fabi

ABUJA, July 6 (Reuters) - Nigeria's main militant group said on Monday it had sabotaged a Chevron (CVX.N) oil facility and seized a chemical tanker and six crew members, the latest in a string of attacks in Africa's biggest energy producer.

The Movement for the Emancipation of the Niger Delta (MEND) said it had attacked Chevron's Okan manifold late on Sunday in the southern Delta state, hours after it sabotaged an oil well head operated by Royal Dutch Shell (RDSa.L). The military confirmed the attack.

"The criminals attacked another pipeline in Delta state yesterday. The pipeline belongs to Chevron," said Colonel Rabe Abubakar, spokesman for the military task force.

U.S. oil firm Chevron, which halted swamp operations in Delta state following attacks on its pipelines in May, said it was investigating the report.

Chevron, Shell and Italian energy firm Agip (ENI.MI) have cut output by around 273,000 barrels per day in the last six weeks following the latest campaign of militant violence.

The disruption to supplies have helped support global oil prices in the past few weeks. But prices fell on Monday to $64 a barrel as traders focused on the global economic recovery. [O/R]

MEND also said on Monday it had seized a chemical tanker and six crew members off the coast of Escravos in the Niger Delta. It said three of the hostages were from Russia, two from the Philippines and one from India.

"Six crew members from the chemical tanker Siehem Peace was seized about 20 nautical miles from Escravos on Sunday ... and will be held until further notice," said MEND, who threatened further offshore attacks.

The military said it was trying to establish contact with the kidnappers. Hundreds of foreigners have been seized in the Niger Delta since early 2006, most of whom have been released unharmed after a ransom was paid.