Russian jets struck 10 ISIS targets in Syria Monday, the Defense Ministry said, as Moscow ramped up its aerial campaign with strikes that targeted the Damascus area for the first time. "Su-34, Su-24M and Su-25 carried out 15 sorties from the Hmeimim air base in the afternoon," the ministry said in a statement, adding that 10 facilities had been struck on the sixth day of Moscow's military intervention in Syria. The raids notably destroyed an ISIS command center and communications hub in a mountainous area of Damascus province, it added. They also destroyed ISIS command centers in the northern province of Aleppo, around 20 tanks and two ammunition depots in the eastern province of Homs, as well as 30 vehicles, including armored vehicles, in northwestern Idlib province. Russian fighter jets launched their bombing campaign in Syria Wednesday in support of the war-torn country's President Bashar Assad. Syria's foreign minister said Monday that Russian airstrikes and its new military "steps" in Syria took months of preparations. Earlier Monday, the Russian Defense Ministry said its jets had struck nine ISIS sites in Syria over the past 24 hours, hitting command centers, weapons caches, artillery and communication posts in the Syrian provinces of Homs, Idlib and Latakia. Meanwhile, the head of the Russian parliament's defense committee said Russian volunteers who have honed their combat skills in Ukraine are likely to travel to Syria to fight alongside the forces of Assad. "It is likely that groups of Russian volunteers will appear in the ranks of the Syrian army as combat participants," Adm. Vladimir Komoyedov told the Interfax-AVN news agency. The Kremlin has said Russia has no current plans to deploy ground troops to Syria and will confine itself to conducting airstrikes to support the Syrian army instead. It has not yet offered a view on the possibility of Russian volunteers or mercenaries fighting in Syria. Moscow insists it is only targeting ISIS militants in Syria but Turkey and its Western allies have accused Russia of attacking moderate rebel forces. Germany Monday voiced doubt about Russia's claims that it was targeting ISIS militants rather than moderate rebels with its airstrikes. "About the ... continuing Russian airstrikes: We would consider it important to reconcile words and actions," Foreign Ministry spokesman Martin Schaefer said. Moscow's unexpected move last week to launch airstrikes in Syria has brought the greatest threat of an accidental clash between Russian and Western forces since the Cold War. Russian warplanes and those of the United States and its allies are now flying combat missions over the same country for the first time since World War II with Moscow repeatedly targeting insurgents trained and armed by Washington's allies. The U.S.-led coalition targeted ISIS with 13 airstrikes in Iraq and eight in Syria Sunday, the Combined Joint Task Force leading the operation said in a statement released Monday. Russian officers will visit Israel Tuesday to discuss how those countries can avoid accidentally clashing while operating in Syria, an Israeli military officer said. Israel has attacked Syrian armed forces and Lebanese Hezbollah fighters during the 4-year-old civil war in Syria. White House spokesman Josh Earnest said there was no sign Russia had changed its strategy to concentrate on fighting ISIS. During a trip to Spain, U.S. Defense Secretary Ashton Carter compared Moscow's effort to bolster Assad to tethering itself to a sinking ship. "By taking military action in Syria against moderate groups targets, Russia has escalated the civil war," Carter said. More than 40 Syrian insurgent groups, including some of the most powerful groups fighting against Assad, called on regional states to forge an alliance against Russia and Assad's other big foreign backer, Iran.