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WASHINGTON (Reuters) - The United States said it was not discussing joint air strikes with Russia on Monday and called on Moscow to press Syrian President Bashar al-Assad’s government to cease air strikes against opposition forces in Aleppo and the Damascus suburbs.

“We’re not looking at joint operations,” said State Department spokesman Mark Toner. “We’re discussing with them proposals for sustainable mechanisms to better monitor and enforce the cessation of hostilities - we’re not talking about joint operations.”

Toner said the United States was concerned about an uptick in violence in Syria - by both Islamic State and Assad’s forces - and said Russia had a special responsibility to press the Syrian leader to end attacks and strikes that kill civilians.

The Syrian government needs to recognize that “if this keeps up, we may be looking at a complete breakdown” of the cessation of hostilities, Toner said. A truce brokered by the United States and Russia in February has been unraveling for weeks.

Washington urged the Assad regime to end its escalating attacks on Aleppo and Daraya, as well as besieging towns and obstructing humanitarian access, the department said.

“Secretary Kerry raised these concerns in a call with Russian Foreign Minister Lavrov earlier today and urged him to press the regime to cease at once airstrikes against opposition forces and innocent civilians in Aleppo and the Damascus suburbs,” the State Department said in a statement.

Islamic State claimed responsibility for bombs that killed nearly 150 people and wounded at least 200 in Jableh and Tartous on Syria’s Mediterranean coast on Monday in the government-controlled territory that hosts a Russian military base.