Officials from the European Union, Russia and Ukraine will meet Monday in Brussels for further talks on the future of Russian gas supplies to Kiev and Europe. They will prepare the ground for a second round of talks at ministerial level later this month, following a meeting last week in Warsaw, the European Commission said Friday. In Warsaw, EU Energy Commissioner Oettinger, Russian Energy Minister Novak and Ukrainian Energy Minister Prodan, agreed that gas supplies would not be interrupted while negotiations continued. Since then, however, the Ukraine crisis has deepened, with pro-Kremlin rebels in the east pressing ahead with plans for a referendum on independence from Kiev on Sunday. On Thursday, Russia said Ukraine will have to pay up-front for all future gas deliveries because of billions of dollars in outstanding bills owed to Moscow. The move was expected but raises the stakes sharply given the 28-nation EU relies on Russia for about 25 percent of its gas supplies and about half of that transits through Ukraine. The European Commission said Monday's talks will continue on the agenda set in Warsaw, covering continued gas transit and supply, outstanding Ukraine debt and the price it pays, plus transparency and reliability of gas flows. The Commission said the next ministerial meeting would take place in mid-May, as agreed in Warsaw, but it would not confirm Russian reports putting it on May 16 in Athens. Monday's meeting coincides with a first trip to Kiev by European Council President Herman Van Rompuy and an EU foreign ministers meeting in Brussels which will discuss imposing possible additional sanctions against Moscow over the crisis in Ukraine. On April 10, Russian President Vladimir Putin sent a letter to 18 EU countries, many heavily dependent on Russian energy supplies, urging immediate negotiations.