UPDATE 2-BP says Russian agents search its Moscow office

(Adds analyst, background)

By Dmitry Zhdannikov and Tanya Mosolova

MOSCOW, May 20 (Reuters) - Russian security service officers were searching the headquarters of oil major BP (BP.L) in Moscow on Tuesday for the second time in the two months, adding pressure on the company amid widely rumoured buyout talks.

BP's Russian venture, TNK-BP, is the subject of long-running market speculation that the Kremlin wants a state company along the lines of gas monopoly Gazprom (GAZP.MM) to buy out the Russian billionaires who own half the company.

"A dozen people came in this morning, the server is down. Some employees were told to switch off their computers, some had to leave the office. They said they were looking for Gazprom-related documents," an industry source told Reuters.

A BP spokesman confirmed searches were taking place at the company, but gave no details.

A spokesman at TNK-BP said his company's offices were quiet, and he knew nothing about the search at BP. The Russian Federal Security Service (FSB) declined to comment.

In March, Russian law enforcement agencies conducted searches at the Moscow offices of BP and at TNK-BP, Russia's third-largest oil producer and which is half-owned by BP.

At that time, a Russian TNK-BP employee was charged with industrial espionage. In both searches, FSB officers were looking for documents related to Gazprom, sources close to the company said.

The Russian billionaire shareholders in TNK-BP have repeatedly denied any plans to sell their stake.

Analysts said Tuesday's search may further strain political relations between Russia and Britain, tense since the 2006 poisoning in London of Alexander Litvinenko, a former Russian agent who had become a critic of the Kremlin.

"This (the search) may relate to another spy scandal, given that there is an undeclared diplomatic war (between Russia and Britain)," said Ekaterina Kravchenko, an analyst at Broker Credit Service, a Moscow-based brokerage.

Russia refused to extradite the man Britain named as chief suspect in Litvinenko's murder, arguing its constitution forbade it. The decision triggered tit-for-tat diplomatic expulsions.

But, whatever the reason for the searches, they make a sale more likely, Kravchenko added.

"As far as I know, TNK-BP's Russian shareholders would agree to sell out. The only issue now would be the price, and things like this surely add pressure and impact its value," she said.

BP relies on TNK-BP, which produces 1.4 million barrels per day, for a quarter of its worldwide production. (Writing by Chris Baldwin and Tanya Mosolova; editing by David Hulmes)