(Updates with woman killed)

By Tom Ashby

LAGOS, Nov 12 (Reuters) - Troops repelled an armed attack on one of Nigeria's largest oil export terminals on Monday and a pregnant woman was killed in the crossfire, authorities said.

It was the third raid on a major oil facility in Africa's largest producer since the arrest of a militant leader in September. Exports from ExxonMobil's <XOM.N> 400,000 barrel-per-day Qua Iboe terminal were not affected.

State-run News Agency of Nigeria said the attackers snatched two machine guns from Navy guards at the terminal and blew up two police boats.

"In the ensuing crossfire, a pregnant woman was killed," it said.

Industry sources estimated the attackers came in eight speed boats.

A spokeswoman for ExxonMobil said the attack was repelled by security agencies and the shooting had stopped by 1200 GMT.

"There was no impact to our facilities or personnel, but I don't know about the security agencies who responded to the attack," Gloria Essien said.

Workers were locked down in secure areas of the terminal, but oil and gas production were not affected. No tanker was loading crude oil at the time, she added.

No group has yet claimed responsibility for the attack.

Industry sources said the militants pulled back, but could launch another assault later. The military sent reinforcements to the area, located on the southeastern coast of Nigeria, they added.

Militants whose attacks have cut Nigerian oil output by one-fifth ended a five-month ceasefire in September when militia leader Henry Okah was arrested on arms trafficking offences in Angola.

The arrest upset a nascent peace initiative by President Umaru Yar'Adua, who came to office in May promising to address grievances of poverty and neglect in the vast wetlands of southern Nigeria. Okah's group, the Movement for the Emancipation of the Niger Delta (MEND), was behind the most crippling attacks on the industry over the past two years and has accused the government of being behind his arrest.

Gunmen kidnapped 13 oil workers from two different offshore oilfields in October. All were released after a few days. MEND claimed responsibility for both abductions.

The government's attempts to lure militants to the negotiating table has split them into factions, and activists in the region say the resurgence of attacks is also a symptom of a struggle for supremacy in militant ranks.

MEND's Okah has been in a war of words with rival militia leader, Mujahid Dokubo-Asari, who has joined the peace talks and spoken out against kidnapping. (Editing by Giles Elgood)

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