UNITED NATIONS (Reuters) - The U.N. Security Council on Monday extended for 18 days the mandate of an international inquiry charged with laying blame for chemical weapons attacks in Syria as Western states on the 15-member council attempt to negotiate a longer renewal.

The year-long inquiry by the United Nations and Organization for the Prohibition of Chemical Weapons has found that Syrian government forces were responsible for three chlorine gas attacks and that Islamic State militants had used mustard gas.

The mandate of the inquiry had been due to expire on Monday, but the council unanimously agreed to extend it until Nov. 18.

France, Britain and the United States hope to persuade Russia to agree to a 12-month extension before starting talks on a draft resolution to punish those blamed for such attacks.

"It is absolutely critical that the (inquiry) gets later a one-year mandate to continue its investigation. We consider it very important," French U.N. Ambassador Francois Delattre told reporters on Monday.

Russian U.N. Ambassador Vitaly Churkin said on Thursday that Moscow would like the inquiry to be strengthened to look more at the "terrorist chemical threat" and to include attacks outside of Syria. Western states have accused Islamic State of using chemical weapons in neighboring Iraq.

Russia has also dimmed Western hopes that U.N. sanctions could be imposed on the Syrian government forces that the inquiry has found responsible for such attacks. Churkin described the findings as "unconvincing" and said they "cannot serve as accusatory conclusions for taking legal decisions."

Syria's government last Wednesday denied its forces had used chemical weapons during the country's nearly six-year civil war.

(Reporting by Michelle Nichols; Editing by James Dalgleish)