European Union and Russian leaders open a two-day summit in the central Russian city of Nizhny Novgorod Thursday that is likely to be overshadowed by a rift over Russia's E. coli-related ban on EU vegetables.

The European Union has reacted angrily to Russia's decision to ban all EU produce. It wants Russia to lift the ban, which the EU says contradicts World Trade Organization rules at a time when Moscow is trying to join the world trade body.

Russian Prime Minister Vladimir Putin has said Russia will not lift the ban until the EU provides details of the source of the E. coli outbreak. But authorities have so far failed to find the source of the virus.

Despite the ban, the sides are hoping to make progress on areas of common concern when Russian President Dmitry Medvedev meets with EU leaders, including European Commission chief Jose Manuel Barroso and EU President Herman Van Rompuy.

In addition to EU-Russia relations, Russia's WTO accession will be high on the summit agenda. Russia has gained the support of the EU in its lengthy bid to join the world trade body, but outstanding issues between the two sides remain, including rules on food imports.

Summit participants also are expected to discuss a new EU-Russia cooperation agreement on trade, energy and investment, the global economy and developments in North Africa and the Middle East.