The EU foreign ministers said in a statement on Monday that "unilateral steps must be avoided, and that all open questions must be resolved through consensual positions" based on Iraq's constitution.
The union also called on all Kurdish parties in Iraq's northern region to "seek common ground" and consider holding regional elections.
In April, the representatives of the two main Kurdish parties of the Patriotic Union of Kurdistan (PUK) and Kurdistan Democratic Party (KDP) discussed the issue of a separation referendum and decided to hold the vote this year.
Masoud Barzani, the president of Iraq's Kurdistan Regional Government (KRG), announced on June 7 that the vote would be held on September 25 in the three governorates that make up the Kurdish region and in the areas that are disputed by the Kurdish and Iraqi governments, including the disputed northern city of Kirkuk, Khanaqin, Sinjar and Makhmur.
The plebiscite has on several occasions been criticized by the central government in Baghdad.
Barzani has repeatedly called for a secession vote, but Iraqi Prime Minister Haider al-Abadi has stressed the need for preserving the country’s territorial integrity.
The premier has also called on the Iraqi Kurdistan to scrap any plans for independence, arguing that the autonomous region could not do without the mainland.
Falling oil prices have severely hit the Kurdish region, which like the capital, Baghdad, heavily relies on oil income to provide the majority of its funds, Presstv reported.
Over the years, Kurdish officials have been at odds with Baghdad over the share of oil income from the northern Iraqi city of Kirkuk as part of the crude produced in the area passes through pipelines operated by the Kurds.