The European Union is expected to bring forward a meeting of foreign ministers due to decide on an oil embargo on Iran by one week to January 23, EU diplomats said on Monday.

They said a final decision would be taken by a meeting of EU ambassadors in Brussels on Tuesday, and the aim would be to avoid overshadowing a summit of EU leaders set for January 30.

"It looks likely it will be brought forward to the 23rd," one of the diplomats said.

EU states have already agreed in principle to an embargo on Iranian oil, part of the latest Western effort to ratchet up pressure on Tehran over its nuclear programme.

However, they still have to finalise details of when it will be imposed. Diplomats say the embargo could take several months to start because some EU capitals want a delay to shield their debt-stricken economies.

EU countries have proposed "grace periods" on existing contracts of between one month and 12 months to allow them to find alternative supplies.

Greece, which depends heavily on Iranian crude, is pushing for the longest delay, the diplomats said. Britain, France, the Netherlands and Germany wanted a maximum grace period of three months.

The goal had originally been for a final decision on January 30, but the decision to move the foreign ministers' meeting will increase pressure for a quicker decision.

A diplomat said an EU working group meeting on Monday had not appeared to narrow the differences.

European measures against Iran's oil industry will complement U.S. sanctions announced on New Year's Eve that aim to make it impossible for most countries' refineries to buy Iranian crude.

Iran is the second-largest producer of oil, after Saudi Arabia, among the 12 countries in Organisation of the Petroleum Exporting Countries (OPEC), producing around 3.5m barrels per day.

EU countries buy about 500,000 barrels per day (bpd) of Iran's 2.6m bpd in exports, making the bloc collectively the largest market for Iranian crude, rivalling China.

The three biggest EU importers have serious debt problems. Greece imports a quarter of its oil from Iran, Italy about 13 percent and Spain nearly 10 percent.