* Syria's foreign reserves have halved -Juppe
* Oil output down 30 pct -diplomats
* Paris to host foreign ministers Thursday
By John Irish
PARIS, April 17 (Reuters) - Western sanctions have halved Syria's foreign currency reserves and should be stepped up to force Damascus to comply with a U.N.-backed peace plan, France's foreign minister said on Tuesday.
"We must maintain pressure on the Syrian regime. That means strengthening sanctions that are having an impact on Syrian authorities," Alain Juppe told officials meeting in Paris to coordinate measures against President Bashar al-Assad.
Foreign ministers will meet in Paris on Thursday, a French official told Reuters, to press Assad to respect a U.N.-backed ceasefire. Ministers from Turkey, Qatar and U.S. Secretary of State Hillary Clinton would be among those attending.
Juppe called for concerted policy: "We will judge the Syrian authorities by their actions. Any violations must be followed by a firm, swift response on the part of the Security Council."
At the conclusion of the meeting, which brought together diplomats and treasury official from 57 countries, those attending agreed to urge more countries to join in efforts to "isolate" the Syrian government and called on businesses dealing with Assad's authorities to break those ties.
"The main message was to enlarge the group and make countries more serious about imposing sanctions," said an Arab diplomat who said some neighbouring countries wanted exemptions from joining in on some measures. "I'm a little disappointed," he said, "Because the international community should do more and shouldn't be leaving the Syrian people to die like this."
Representatives of the Syrian opposition were also present. Known as the "sanctions working group", the officials will meet again next month in Washington.
International divisions over how to deal with the Syrian crisis has seen sharp debates over sanctions.
"Certain countries are unambiguously signaling their support for the Syrian regime, others are more or less directly offering alternative deals," Juppe told the meeting.
In Moscow, Russian Foreign Minister Sergei Lavrov, whose government has been a key defender of Assad in the U.N. Security Council, indirectly criticised Western and Arab states whose support for Syrian rebels was, he said, obstructing a dialogue within the country that could resolve the crisis.
Juppe said Syria's foreign reserves had been cut in half by the sanctions - led by the European Union and United States. The reserves were estimated at $17 billion before protests began 13 months ago which prompted a government crackdown.
Diplomatic sources estimated the sanctions had cut Syria's oil output by 30 percent, costing Assad's government $400 million a month in revenue, or $2 billion since November. Prior to EU sanctions, Damascus sold 90 percent of its oil to Europe.
The Syrian pound hit a record low on the black market in March of around 100 to the dollar, compared to 47 before the protests erupted, sharply raising the cost of imports.
Diplomats said that sort of economic difficulty would over time increase the pressure on Assad's government.