The US government has held back from congratulating Romano Prodi as Italy's next leader, saying the election process is not over yet.
The official results have given the centre-left opposition leader a narrow victory over Prime Minister Silvio Berlusconi's centre-right government.
Mr Berlusconi has alleged irregularities in the election.
The White House has said it is waiting until a recount of the disputed votes due to be completed later this week.
A spokesman said the US would work with whatever government was elected.
However, President George W Bush has had a close working relationship with Mr Berlusconi, who strongly supported the war in Iraq.
Reports say the election of Mr Prodi as Prime Minister could bring changes in Italy's relationship with the US.
He opposed the war in Iraq and favours a more united Europe as a counter-balance to US power.
