LONDON (Reuters) - Britain said on Sunday Russia bore responsibility by proxy for civilian deaths in Syria last week caused by a poison gas attack that Washington says was carried out by the Moscow-backed government of Syrian President Bashar al-Assad.

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At least 70 people died in what the United States says was a chemical weapons attack in rebel-held Syria. The attack prompted the United States to fire 59 cruise missiles into a Syrian air base from which it said the attack was launched.

Damascus and Moscow denied Syrian forces were behind the gas attack but Western countries dismissed their explanation that chemicals leaked from a rebel weapons depot after an air strike.

Russia has warned that the U.S. missile strikes could have serious consequences for the region. The missile strikes catapulted Washington into confrontation with Russia, which has advisers on the ground aiding its close ally Assad.

British Defence Secretary Michael Fallon criticized Russia’s support of Assad, describing the chemical attack as a war crime that happened “on their watch”.

“By proxy Russia is responsible for every civilian death last week,” Fallon, whose government voiced support for U.S. President Donald Trump’s decision to target the Syrian air base, wrote in the Sunday Times newspaper.

“If Russia wants to be absolved of responsibility for future attacks, (President) Vladimir Putin needs to enforce commitments, dismantle Assad’s chemical weapons arsenal for good and get fully engaged” with the U.N. peace process on Syria.

On Sunday, Russia’s Foreign Ministry criticized a decision by British Foreign Secretary Boris Johnson to cancel a visit to Moscow later this month, saying it showed a lack of understanding of events in Syria.

The ministry said it also showed once more that there was little to gain from talking to Britain, which it said had no real influence over world affairs.

Washington has long backed rebels fighting Assad in a multi-sided civil war that has killed more than 400,000 people and driven half of Syrians from their homes since 2011.

The United States has conducted air strikes against Islamic State, which controls territory in eastern and northern Syria, and a small number of U.S. troops are helping rebel militias.