The decision brings to 48 the number of people that the EU has put under sanctions for, it says, helping undermine Ukraine's territorial integrity.
The names of the 15 will not be made public until they are published in the EU's Official Journal on Tuesday.
EU diplomats said they would not include the heads of Russian energy giants such as Rosneft's Igor Sechin, who was included on a new U.S. sanctions list on Monday.
The EU has so far only put sanctions on individuals, not companies. EU ambassadors, meeting on Monday, discussed the need to broaden the legal basis to enable the bloc to put sanctions on companies, diplomats said.
The EU is split between countries in favor of stronger action, including Britain, France, Poland, Sweden,Denmark, the Czech Republic and the Baltic countries, and those who are reluctant, such as Italy,Greece, Cyprus, Bulgaria, Hungary, Luxembourg, Austria, Spain, Portugal and Malta.
"The main issue is to see how best to get a diplomatic resolution to the crisis, what is necessary to entice Russia to sit around the (negotiating) table. Sanctions are not an end in themselves," one diplomat said.
"I can't imagine that if the Americans act that the Europeans won't do anything," he said.