The EU, which has so far implemented six rounds of sanctions since Russia invaded Ukraine on Feb. 24, needs unanimity to agree any such ban that Belgium - home to the world's biggest diamond trading hub Antwerp - has rejected in the past.

The bloc was spurred into fresh action after President Vladimir Putin ordered a partial military mobilisation last week and moved to annex parts of eastern Ukraine.

The EU's executive European Commission is expected to present a formal proposal for more sanctions to member states this week.

Poland, Ireland, Lithuania, Estonia and Latvia proposed introducing a retaliatory ban on imports of diamonds from Russia, where Alrosa is the world's largest producer of rough gems.

The Antwerp World Diamond Centre said sanctions would take away some 30% of business and benefit rival trade hubs, adding clients should be allowed to decide themselves if they wanted Russian gems.

Belgium's foreign ministry and the diplomatic representation to the EU did not respond to requests for comment.

But on Sept. 14, Prime Minister Alexander de Croo told an international diamonds conference that such a ban would be a "huge loss" and would hurt an industry accounting for 5% of Belgium's exports and supporting some 30,000 jobs.

An EU official and a diplomat involved in preparations of new measures against Russia said, however, that Belgium was now expected to lift its veto.

Following the Commission's proposal on new sanctions, diplomats and ministers will negotiate until they reach consensus, failing which the matter goes to the 27 national leaders when they meet in Prague on Oct.6-7.

(Writing by Gabriela Baczynska; Editing by Emelia Sithole-Matarise)

By Gabriela Baczynska and Philip Blenkinsop