* Two Russians, a Lithuanian still missing
* Attack marks shift south in West African piracy
* Gulf of Guinea growing source of oil to U.S.
(Adds details on Lithuanian vessel in paragraphs 2 and 4)
By Richard Valdmanis
DAKAR, May 19 An attack by gunmen on two ships anchored off Cameroon's major port of Douala shows pirates are extending their range in the Gulf of Guinea, an increasingly important source of oil to western markets.
Two Russian nationals from the cargo ship North Spirit and a Lithuanian captain from another vessel, Argo, were seized in the back-to-back raids May 16, according to the Russian Seafarer's Union and the owner of Argo.
RSU spokesman Vadim Ivanov said there had been no contact as yet with the attackers.
Lithuania-based Limarko Shipping LLK1.VL, which owns the Argo refrigerated vessel, said in a statement that gunmen robbed the ship's safe and abducted the captain.
Cameroonian authorities were not immediately available to comment.
The raid was the latest in a string of pirate attacks in the Gulf of Guinea -- which stretches from the Guineas in the northwest to Angola in the south -- and marked a shift beyond the Cameroon-Nigeria maritime frontier where most attacks have been clustered.
"There seems to be a pattern emerging. They are moving south," said Rolake Akinola, Africa analyst at Eurasia Group. "It is obviously a real concern. Douala is the hub for the CEMAC (Central African Economic and Monetary Community) region. Insecurity appears to be following increased investor activity."
Douala's port serves land-locked Chad and Central African Republic, as well as some parts of the two Congos.
Cameroon last month blamed piracy for part of a 13 percent slide in oil production in 2009. The country's output averaged 73,000 barrels per day last year, down from 84,000 bpd in 2008.
The Gulf of Guinea is also home to major oil producers Nigeria, Angola, Gabon, and Equatorial Guinea as well as soon-to-be exporter Ghana.
Attacks in the Gulf of Guinea are not on the scale of those off Somalia, where pirates are earning tens of millions of dollars from seizing merchant vessels, but analysts say the insecurity off West Africa could affect shipping and investment.
"Oil companies won't stop doing business because of piracy but we may see much more caution," Akinola said. "It could exacerbate an already downward trend of production."
The U.S. military is training West and Central African navies and coast guards to combat piracy, drug smuggling and illegal fishing in the Gulf of Guinea -- a region Washington estimates will supply a quarter of U.S. oil by 2015.
Ivanov said the seized Russians included chief engineer Igor Shumik, who was among the 15 sailors aboard the Arctic Sea vessel that vanished for nearly a month last summer and which Russian authorities said had been hijacked by pirates near Sweden before being intercepted by the Russian Navy.
The other missing Russian is Boris Tersintsev, the North Spirit's captain, Ivanov said.