Charles Dallara, managing director of the Institute of International Finance (IIF).

Cyprus, rather than Spain, Italy or Greece, poses the biggest sovereign risk to the euro zone, according to Charles Dallara, the managing director of the Washington-based bank lobby group, the Institute of International Finance (IIF).

"I think Cyprus is, on a country level, the most serious risk the euro zone faces today, not Spain, not Italy, not Greece," Dallara told CNBC in Switzerland on Tuesday.