US official announced that his country refuses the proposal made by Russia for joint strikes in Syria.
Earlier, The Russian defense minister proposed on Friday that Russia and the U.S.-led coalition launch joint action against al-Qaida's branch in Syria, known as the Nusra Front.
Defense Minister Sergei Shoigu said that Moscow had proposed to Washington that the coalition call on all factions eager to abide by a cease-fire in Syria to leave the areas where the al-Qaida branch is active by May 25.
Moscow suggested that starting from that date, Russia and the U.S.-led coalition conduct joint strikes against the Nusra Front and any other groups refusing to honor the truce, Shoigu said.
However, the Russian minister warned that Moscow reserves the right to unilaterally strike militants refusing to respect the cease-fire, as well as weapons and militants crossing into Syria from Turkey, starting from May 25.
Russia long has accused Turkey of serving as a conduit for weapons and supplies flowing to the Islamic State and the Nusra Front - claims which Ankara has denied.
Shoigu said the Russian proposal should help secure the cease-fire, adding that Moscow has coordinated it with Damascus. He claimed that the Russian military has started discussing the idea with the U.S. representatives.
Coalition flyers urge residents to quit ISIS Syria bastion
Meanwhile, the US-led coalition fighting the ISIS group has dropped flyers on Syria's Raqqa urging residents for the first time to leave the jihadist stronghold, an activist and a monitor said Friday.
"It's not the first time that coalition airplanes have dropped flyers on Raqqa but it's the first time that they've addressed residents and asked them to leave," a founding member of the citizen journalist group Raqqa is Being Slaughtered Silently (RBSS) said.
Flyers usually address ISIS fighters in the city, telling them "Your time is coming, your end is coming", Abu Mohammad said.
On its Facebook and Twitter accounts, RBSS shared an image of what they said was the flyer dropped on the militants' de facto capital in Syria.
"The time you have been waiting for has come, the time has come to leave Raqqa," reads the top of the comic book style water-colored sketch.
It shows residents -- three men wearing backpacks, a veiled woman and a child -- fleeing a grey battered city into an idyllic green countryside, running past dead soldiers and ISIS fighters and a sign marked: "IS, Raqqa province, checkpoint".
Abu Mohammad said that ISIS fighters in Raqqa were increasingly using residents as human shields against coalition air strikes.
"Their positions used to be clear but since the beginning of the air campaign against them, they've started to resort to hiding among civilians," he said.
At least 408 civilians have died in coalition air strikes in Syria since they began in 2014, according to the Syrian Observatory for Human Rights.
"It's the first time residents have been advised to leave the city," Observatory chief Rami Abdel Rahman said.
But "these flyers are just a part of the propaganda campaign against IS," he added.
Kurdish forces, which control a swathe of territory north of Raqqa, are regarded by Washington as the most effective fighting force against ISIS in Syria and have received US military support.
"There has been information circulating about the Kurds preparing a campaign against IS in Raqqa with the support of the international coalition," Abdel Rahman said.
But "that's not likely now because (retaking) Raqqa will require planning a massive battle, and huge preparation of fighters and popular support," he said.
"If the extremist group had feared a battle against it in Raqqa, it wouldn't have sent its best fighters to take part in fighting in the north of Aleppo province."
ISIS and its militant rival Al-Qaeda are not party to a February ceasefire between government forces and other rebel groups.
Air strikes against them have continued, not only by the coalition but also by the Syrian government and its ally Russia.
with AP and AFP
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