By Denis Pinchuk

MOSCOW (Reuters) - Exxon Mobil <XOM.N> Chief Executive Rex Tillerson is to attend a major investment forum in Russia this week, two industry sources said, ending a two year absence from the event that began when sanctions were imposed on Russia over the crisis in Ukraine.

Tillerson is due to attend a session at the St. Petersburg International Economic Forum where Igor Sechin, head of Russian state oil firm Rosneft <ROSN.MM>, is listed as keynote speaker. Both Rosneft and Sechin are subject to Western sanctions.

A spokesman for Exxon Mobil declined to comment on whether Tillerson would be at the June 16-18 forum, which will also be attended by Russian President Vladimir Putin.

The annual forum is the marquee event in Russia's business calendar where investors seek the Kremlin's blessing for their projects. In the past few years, the guest list became a barometer of international investor appetite for Russian assets.

The number of high-profile Western investors and politicians coming to the event dropped off sharply in 2014, after Russia annexed Ukraine's Crimea Peninsula and backed pro-Moscow separatists in eastern Ukraine.

This year, the numbers are recovering. Italian Prime Minister Matteo Renzi is to attend, along with European Commission President Jean-Claude Juncker.

Western diplomats say businesses are looking to Russia again in anticipation that the sanctions will be dropped or softened within the next year or so. They are also attracted by tentative signs the Russian economy is emerging from its slump.

Tillerson's presence is particularly significant because he will be the most high-profile representative of U.S. business at a time when Washington is taking a harder line than Europe on the need to keep sanctions in place.

Tillerson is among other oil major chief executives on the forum agenda, including Robert Dudley of BP <BP.L>, Claudio Descalzi from ENI <ENI.MI>, Schlumberger’s <SLB.N> Paal Kibsgaard and Patrick Pouyanne of Total <TOTF.PA>.

In 2014 Exxon was forced by the sanctions to suspend its joint operations with Russia's top oil producer Rosneft ROSN.MM in the offshore Arctic Kara sea oil field, where they successfully performed exploration drilling.

The sanctions prevent Western companies from giving Russia technology for Arctic offshore, deepwater and shale oil development. After Exxon stepping back, Rosneft has not resumed drilling in the Arctic Kara Sea, citing the sanctions.

Exxon has also been involved in a joint oil and gas project with Rosneft, Sakhalin-1, which produces over 100,000 barrels per day of oil. Sakhalin-1 is not subject to the sanctions.

In addition, Exxon and Rosneft are discussing a joint project to build a liquefied natural gas plant in Russia's far east.

Tillerson was last in Russia in 2015 for a meeting with Russian Energy Minister Alexander Novak. He missed the St Petersburg forum in 2014 and 2015.

According to the forum's Internet site, Tillerson attended the event for two consecutive years before the Ukraine crisis.

(Additional reporting by Katya Golubkova; Writing by Christian Lowe; Editing by Alexandra Hudson)