EU nations agreed Thursday to slap sanctions on Iran's central bank, freezing assets potentially used to finance its nuclear drive, but have not yet clinched an oil embargo deal, EU diplomats said.

"The text was considered closed and agreed," a source said of the financial sanctions after talks between senior diplomats from the 27 EU nations.

Diplomats have held lengthy meetings through the week in Brussels to agree an oil and financial embargo against Tehran that is to be formally adopted at talks between European Union foreign ministers on Monday.

The freeze on assets held by Tehran's central bank is to be partial, "enabling legitimate trade to go ahead", while ensuring continued payment of outstanding Iranian debts to Europe, an EU diplomat said.

"The idea is to dry up sources of finance for Iran's nuclear programme," said the source, who spoke on condition of anonymity.

But the high-level diplomats finessing the action on Iran were unable to clinch details on an oil embargo, the source added.

"They'll have another go on Monday morning" ahead of the foreign ministers' meeting, "but it is likely that the oil embargo will have to go directly to the foreign affairs council" of ministers, said another diplomat.

The new EU sanctions are part of a concerted effort with the United States to pressure Iran into halting its controversial nuclear activities, which the West suspects are aimed at developing nuclear weapons.

The EU imported some 600,000 barrels of Iranian oil per day last year, according to the International Energy Agency, making it a key market alongside China, which has refused to bow to pressure from Washington, and India.

But Greek concerns are holding up a final deal on the oil embargo.

"The issue is how to reassure the delegation that depends most on Iranian oil that all is done to protect them," said a diplomat.

Iranian oil accounted for 34.2 percent of Greece's total oil imports, 14.9 percent of Spain's and 12.4 percent of Italy's in the first nine months of last year, according to the latest EU statistics.

Saudi Oil Minister Ali al-Naimi said this week that the kingdom's output could be boosted by around 2.6 million barrels a day to offset a potential cut in Iranian exports.

But Iran warned Saudi Arabia to reconsider its vow to make up for any shortfall.