FRANKFURT—The European Central Bank is now open to purchasing government bonds of Italy and Spain, though has made no firm commitment to do so, according to people familiar with the matter.

Buying bonds of the two countries, which together issue roughly €600 billion ($857 billion) of government bonds a year, would be a major step for the central bank. Until this week, the ECB had bought less than €80 billion of Greek, Portuguese and Irish government bonds.

ECB...