NEW : President Obama sends Vladimir Putin letter addressing the matter . U.S. believes Russia has tested cruise missiles not allowed by 1987 treaty . Comment: Source tells CNN that Washington has proposed senior-level talks with Moscow .
NEW : President Obama sends Vladimir Putin letter addressing the matter . U.S. believes Russia has tested cruise missiles not allowed by 1987 treaty . Moscow says it investigated and considers the matter closed . Source tells CNN that Washington has proposed senior-level talks with Moscow .