* Political row over privatisation deals
* NIS deal brokered by party now in opposition
* Government facing scrutiny over airline sale (Updates with Gazprom Neft reaction)
By Ivana Sekularac
BELGRADE, Aug 12 (Reuters) - Serbia is to launch an investigation into the 2008 privatisation of state oil company NIS, now majority-owned by Russia's Gazprom Neft , the country's interior ministry said.
Gazprom Neft, which bought a 51 percent stake in the NIS monopoly for 400 million euros ($534 million) in 2008, said it was surprised that the Serbian authorities would bring "our fruitful work into question".
In a statement late on Monday, the interior ministry said it would investigate the deal that was brokered under a government led at the time by the Democratic Party, which is now in opposition.
Some politicians at the time said the sale grossly undervalued NIS. It coincided with the climax of a drive by Serbia's former southern province of Kosovo to secede, with Serbia relying on ally Russia's veto in the Security Council to block any U.N. endorsement of a declaration of independence.
"A special investigative team will examine all the facts and circumstances regarding the privatisation of NIS," the ministry said.
The sale was part of a wider energy deal between Russia and Serbia, which included a commitment to build the South Stream pipeline through Serbia and construction of a gas storage facility that has since been completed.
Serbia is in the middle of a dispute between the European Union, which it wants to join, and Russia's South Stream project, with the EU effectively blocking construction in Serbian neighbour Bulgaria.
"Our relations with the government have always been based on partnership," Gazprom Neft First Deputy CEO Vadim Yakovlev said during a conference call on Tuesday.
"We don't see the logic of bringing our fruitful work into question. We hope to find a solution through direct dialogue (with the government)."
Announcement of the investigation comes as the ruling Serbian Progressive Party (SNS) of Prime Minister Aleksandar Vucic faces calls to publish the terms of a deal last year under which United Arab Emirates airline Etihad took a 49 percent stake in Serbia's indebted flag carrier JAT Airways, now Air Serbia.
Vucic spearheaded that deal and promised to publish the contract, but the government has yet to do so. On Tuesday he said it would be released to the public on Thursday.
A Democratic Party official denied that the NIS sale was "damaging" for Serbia.
"The agreement was ratified by parliament," Borislav Stefanovic, a senior party official, told the Beta news agency.
Gazprom Neft has since increased its share in NIS to 56.15 percent. Almost 30 percent is owned by the Serbian government and the rest by small shareholders.
NIS runs two refineries in Serbia and produces oil and gas totalling 1.7 million tonnes of oil equivalent a year, operating fields in Serbia, Angola and Bosnia.
A spokeswoman for NIS said: "The matter should be resolved between the two countries." (1 US dollar = 0.7485 euro) (Additional reporting by Olesya Astakhova in Moscow; Editing by Matt Robinson and David Goodman)