Washington DC - International election observers have criticized the US for failing so far to invite them to monitor November's crucial midterm elections, which will determine the balance of power in Congress.

The Organization for Security and Co-operation in Europe (OSCE) on Wednesday said the lack of an invitation was "regrettable," particularly given its "long-standing co-operation with the country in the field of election observation."

The White House did not immediately respond to a request for comment.

The OSCE has observed 12 federal elections in the US since 2002, including six midterm elections, the organization said.

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The November 3 vote would be the first since the OSCE began monitoring elections in the country for which it has not received an invitation.

The US decision represents "a significant departure from an established democratic practice," said Pere Joan Pons, president of the OSCE Parliamentary Assembly.

An observation mission would have provided a valuable independent assessment of changes to the legal and regulatory framework since the last federal elections, the OSCE said.

The organization frequently monitors elections in established democracies.

The 57-country OSCE sees itself as an East-West platform for managing conflicts and defending democracy and human rights. Its work includes monitoring elections and conflict zones.