UPDATE 1-Turkey's Oyak to buy Gemport from Isbank-source

* Stake valued at $350-400 mln - analyst

* Port is Turkey's 10th biggest

(Adds comment, details) By Birsen Altayli

ISTANBUL, Oct 2 (Reuters) - Turkish army pension fund Oyak has agreed to buy Isbank's port unit Gemlik Liman (Gemport), a source familiar with the matter told Reuters on Tuesday, in a deal which an analyst estimated was worth $350-400 million.

Isbank said late on Monday it had signed a deal to sell its 54 percent stake in Gemport but gave no further details.

"We estimate a roughly $350 million-$400 million transaction value," Oyak Securities, a unit of the pension fund, said in a research note following Isbank's announcement.

Oyak declined to comment. Isbank was not immediately available to comment.

Gemport is located near Istanbul, south-east of the Marmara Sea, and has an annual container storage capacity of 600,000 twenty-foot equivalent units (TEU), according to its website. TEU is a standard unit for the cargo capacity of container ships and container terminals.

Pension funds are increasingly investing in infrastructure assets, like ports, attracted by their stable, long-term returns and keen to diversify amid volatile financial markets.

Gemport was the 10th biggest port in Turkey in terms of handling capacity as of 2009, according to an Ernst and Young report.

Founded in 1961, Oyak is Turkey's first and largest privately-owned pension fund, with more than 260,000 members.

Turkey has a coastline of more than 8,000 km and is strategically positioned between Europe and Asia. The country has around 175 ports, mostly privately operated and concentrated around Istanbul, its largest city.

Turkey does 88 percent of its trade via its ports, according to Ernst and Young. Its overall handling capacity was 6.6 million TEU in 2011, three times that of Greece, according to independent market monitor Drewry Shipping Consultants.

(Writing by Seda Sezer; Editing by Nick Tattersall and Mark Potter)

((seda.sezer@thomsonreuters.com)(+90 212 350 7051)(Reuters Messaging: seda.sezer@thomsonreuters.com))