In a second day of raids in Syria, Russian warplanes carried out a new round of airstrikes on Thursday that once again — contrary to Moscow’s assertions — appeared to be targeting not the Islamic State but a rival insurgent coalition. Russia sent more than 50 aircraft on about 30 sorties over Syria on Thursday, using drones and satellites to identify targets, said Maj. Gen. Igor Konashenkov, a spokesman for the Russian Defense Ministry. They were able to deploy quickly, he said, because ammunition and other supplies had been stockpiled at the Tartus naval base on Syria’s Mediterranean coast, Russia’s only military site outside the former Soviet Union. Russia’s entry into the Syrian conflict, which started on Wednesday with a bombing attack on Syrian opposition fighters, has been angrily condemned by United States officials. They fear that President Vladimir V. Putin of Russia is using their shared goal of defeating the Islamic State as a pretext for weakening other opponents of Syria’s embattled president, Bashar al-Assad. Mr. Putin says that Mr. Assad is a bulwark against the Islamic State, also known as ISIS or ISIL; President Obama says that Mr. Assad must go, though perhaps in a “managed transition” to a new government.