Syrian army jets carried out at least 25 air strikes on the Islamic State-held city of Palmyra on Friday, a group monitoring the war said, the second intense bombardment in two days of territory held by the militants.
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The Syrian Observatory for Human Rights said it was one of the most sustained government bombardments of Palmyra.
The air strikes killed at least 26 people, including 12 Islamic State fighters, the British-based Observatory said.
On Thursday, Syrian jets had carried out at least 12 air strikes on Raqqa, Islamic State's de facto capital in the north.
A source told Reuters this week that Syrian forces had started using new types of "very accurate" air and ground weapons supplied by its ally Russia.
The prospect of greater military involvement by Russia has alarmed the United States, which is leading a coalition that has been bombing Islamic State strongholds in both Syria and Iraq.
Washington rejects the idea, advocated by Moscow, of cooperating with Syrian President Bashar Assad to fight Islamic State, and has warned Syria not to interfere in its air campaign.
However, the White House did open the door on Thursday to possible tactical discussions with Moscow, and the Pentagon said such talks might be necessary to avoid "miscalculation".
On Friday, U.S. Secretary of State John Kerry said President Barack Obama saw military talks with Russia on Syria as an important next step and hoped they would take place soon.
"Our focus remains on destroying ISIL (Islamic State militants) and also on a political settlement with respect to Syria, which we believe cannot be achieved with the long-term presence of Assad," he told reporters. "We're looking for ways in which to find a common ground."