By Michelle Nichols

UNITED NATIONS (Reuters) - The U.N. Security Council on Thursday extended for one year an international inquiry to determine blame for chemical weapons attacks in Syria, paving the way for a showdown between Russia and Western powers over how to punish those responsible.

Russia had said it wanted the inquiry to be broadened to look more at the "terrorist chemical threat" within the region and the resolution to renew the mandate included language to reflect that request.

The 15-member council unanimously adopted the U.S.-drafted resolution.

Launched by the Council a year ago, the inquiry by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) has already found that Syrian government forces were responsible for three chlorine gas attacks and that Islamic State militants had used mustard gas.

Syria's government has denied its forces have used chemical weapons during the country's nearly six-year-old civil war, while Islamic State is not known to have commented. The Damascus government and its main Russian ally refer to all rebels fighting to topple President Bashar al-Assad as "terrorists".

France, Britain, the United States and other Council members have said that after the renewal of the inquiry on Thursday, they hope to start negotiations on a draft resolution to punish those blamed for the attacks, likely with U.N. sanctions.

French Foreign Minister Jean-Marc Ayrault said on Friday that nobody could deny the Syrian government's culpability and that a resolution needed to be brought to the Security Council.

"The first conclusions from investigators were damning. It is impossible to deny that the Syrian government repeatedly used chemical weapons in clear violation of international law and commitments it agreed to. Those responsible must now be held accountable," he said during a news conference alongside U.N. Secretary General Ban Ki-moon.

But Russia has said the inquiry's findings cannot be used to take action at the Security Council and that the Syrian government should investigate the accusations.

Last week, the OPCW's executive body voted to condemn the use of banned toxic agents by the Syrian government and Islamic State jihadists.

Chlorine's use as a weapon is prohibited under the Chemical Weapons Convention, which Syria joined in 2013. If inhaled, chlorine gas turns to hydrochloric acid and can kill by burning lungs and drowning victims in the resulting body fluids.

Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Moscow and Washington. The Security Council endorsed that deal with a resolution that said in the event of non-compliance, "including unauthorized transfer of chemical weapons, or any use of chemical weapons by anyone" in Syria, it would impose measures under Chapter 7 of the U.N. Charter.

Chapter 7 deals with sanctions and authorization of military force by the Security Council. It would need to adopt another resolution to impose targeted sanctions - a travel ban and asset freeze - on people or entities linked to the attacks.

(Additional reporting by John Irish in Paris; Editing by Mark Heinrich)