* EU opens door to Syrian oil sales to Europe
* Hopes to provide financial support to Syrian rebels
* Opposition at least month away from ability to market oil (Adds quotes from Ashton and Barroso, details)
By Justyna Pawlak and Adrian Croft
LUXEMBOURG, April 22 (Reuters) - European Union governments agreed on Monday to ease sanctions on Syria to allow purchases of oil from the opposition, in the hope of throwing a financial lifeline to rebels fighting President Bashar al-Assad.
The decision, taken at a meeting of EU foreign ministers in Luxembourg, will allow European importers to buy crude oil from Syria, if authorised by an opposition umbrella grouping.
The sanctions were imposed in 2011 in response to Assad's brutal crackdown on pro-democracy protests. Two years later, the conflict is largely a stalemate, and an estimated 70,000 people have died.
EU officials said the easing of oil sanctions would be followed by more aid and other support for the rebels, amid mounting fears of a humanitarian disaster throughout the region.
"The humanitarian situation is extremely alarming, the council (of ministers) today adopted a decision that will allow the Syrian National Coalition to take advantage of the oil and gas reserves under its control," the EU's foreign policy chief, Catherine Ashton, told reporters.
The new rules will also allow European companies to resume investment in Syrian oil infrastructure, provided the cash goes to the rebels, and sell them equipment related to the sector.
Buying Syrian crude will be complicated because of security concerns and battered infrastructure, but officials said more financial help would be offered.
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A prominent member of the Syrian opposition said on Monday it would take at least another month before the group could sell crude, largely because it still does not have a provisional government to oversee possible sales.
Activists say Islamist rebels also are clashing with tribesmen in eastern Syria over the oil facilities in the power vacuum left by the civil war.
The latest U.S. government data indicate oil production in Syria was 153,000 barrels per day in October 2012, a nearly 60 percent decline from the start of the conflict in March 2011.
"It is important for us to send a signal that we are open to helping in other ways, in all the ways possible, including ways adding to the finances (of the opposition)," British Foreign Secretary William Hague told reporters in Luxembourg.
In Brussels, the head of the EU's executive Commission, Jose Manuel Barroso, said after a meeting with U.S. Secretary of State John Kerry, visiting for a NATO meeting, that the EU would give more humanitarian aid for Syria.
"The European Union is preparing a very comprehensive package of support in the humanitarian field, because of the refugees that we have seen increasing to unbearable numbers with unbearable suffering," he said.
More than 4 million people are internally displaced in Syria and nearly 1.4 million have sought refuge in neighbouring countries.
The Syrian crisis risks unsettling Lebanon and causing a humanitarian catastrophe in Europe's backyard, the EU's humanitarian chief said on Monday, calling for a new drive to help refugees and strained neighbouring states.
EU governments remain divided on other support for the rebels, with Britain leading a contested push to ease the bloc's embargo on sending arms to Syria. (Editing by Alison Williams)