BRUSSELS, June 04. /ITAR-TASS/. The crisis in Ukraine and relations with Russia will be the main subjects of discussion at a meeting of the leaders of seven European countries. US President Barack Obama arrives in the Belgian capital to attend the meeting which opens on Wednesday.

It is already a second time this year that Obama visits Brussels, whereas in the rest of his presidency he did not find time for that. Obama's previous trip here was made on March 25-26 when the European Union (EU) - US planned summit was held.

In order to ensure safety of the American leader's visit and the summit, Belgium has introduced a temporary control on its borders, which is an exceptional measure for an EU state. In accordance with the EU legislation, a temporary restoration of border control in Schengen Zone countries is possible in special cases when a threat to state security arises. The Belgian authorities resort to this enforced measure for the first time.

Enhanced security measures have been taken in the capital in view of the terrorist act, which was committed at the Jewish Museum of Brussels on May 24 and in which four people were killed.