DAVOS, Switzerland — Exxon Mobil Corp. signed a deal with Russia's Rosneft on Thursday to develop oil and gas resources in the Black Sea, a new boost for the country's lucrative energy sector despite concerns about the challenges of investing there.

Exxon Mobil Chairman and CEO Rex Tillerson announced the deal with Rosneft Chairman Igor Sechin, who is also Russia's deputy prime minister, at the World Economic Forum in Davos, Switzerland.

They did not immediately disclose the potential value of the deal, but said it could lead to crude oil sales to Rosneft's Tuapsinsky refinery and other Black Sea markets.

The agreement gives Exxon access to Russian resources and Russia access to Exxon's technology. They will explore an 11,200-square-kilometer (4,300-square-mile) deep water offshore area in Russia's sector of the Black Sea.

"It's an important opportunity to expand our activities to new areas now," Tillerson told reporters at the announcement.

Sechin hailed the deal as a sign that Russia is open for investment.

Concerns about Russia's legal problems, state interference and security — especially after a deadly airport bombing Monday — have surfaced at the Davos forum. Russian President Dmitry Medvedev gave a speech Wednesday night in which he insisted that investors shouldn't be scared away.

Last week oil giant BP Ltd. signed a deal with Rosneft to exploit the Arctic Sea, and BP chief Bob Dudley insisted that he was confident about the investment climate in Russia despite his company's difficulties with Russian investments in the past.

But on Thursday, BP announced it's been hit with another challenge: The company was notified by a London court about a lawsuit put forward by AAR, Russian shareholders of BP's Russian venture TNK-BP.

BP's spokesman in Russia, Vladimir Buyanov, told The Associated Press that the shareholders claim BP's recent deal with Rosneft violates a shareholders' agreement between AAR and BP.

"We are not in breach of the shareholder agreement. We have kept, and will keep, AAR informed of the situation ... and will fully meet our obligations under the shareholder agreement to inform them. We will seek a business-like resolution to this situation," BP said in a statement.

Exxon Mobil, meanwhile, had been on the verge of buying a large share of Russian company Yukos before its chief, Mikhail Khodorkovsky, was jailed in 2003, in a case that has stained Russia's investment reputation for the past eight years.

Khodorkovsky has accused Sechin of initiating the legal onslaught against him and Yukos, once Russia's largest and fastest growing oil company. Yukos' prime assets were passed to Rosneft — now Russia's largest oil company itself.

Some of the world's biggest companies have billions in investments in Russia and have weathered the difficulties in exchange for access to a large and lucrative — if volatile — emerging market.

Russian authorities vaunted a new example of this Thursday: Anti-monopoly authorities gave the go-ahead for Pepsico Inc. to buy a majority stake in Wimm-Bill-Dann Foods, Russian Deputy Prime Minister Igor Shuvalov announced in Davos. The $3.8 billion deal unveiled last month creates Russia's biggest food and beverage company, and is Pepsi's largest international acquisition ever.

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"We love Russia," Pepsico Chair and CEO Indra Nooyi told a panel at Davos on Thursday.

In a less successful attempt at attracting investment, the Russian government unveiled a plan Wednesday night to build five ski resorts in the majestic, largely uncharted slopes of the Northern Caucasus.

The problem: The presentation came two days after a bomb blast pummeled the international arrivals terminal of Moscow's largest airport, killing 35. Suspicions immediately fell on extremists from the Northern Caucasus — including Chechnya, which fought two wars with Moscow over the last 16 years, and the neighboring Russian republic of Dagestan, future home to one of the new resorts.

Nataliya Vasilyeva in Moscow contributed to this report.