MOSCOW — Russian jets carried out a second day of strikes in Syria Thursday, and some activists claimed that the targets incluwded rebels backed by the United States as concerns grew about a conflict that has now drawn in warplanes from the world’s two most powerful militaries.

Russian President Vladimir Putin denied reports that civilians were killed in any Russian airstrikes.

“We are ready for such information attacks,” he said in a live broadcast from the Kremlin. “The first reports of civilian casualties came even before our jets took off.”

Russian Defense Ministry Igor Konashenkov said Russian aircraft damaged or destroyed 12 targets in Syria belonging to the Islamic State group including a command center and two ammunition depots. Officials acknowledged, however, that other unidentified groups were being targeted as well.

Konashenkov said Russian Su-25M and Su-25 jets flew 20 sorties between Wednesday and Thursday morning, and he insisted that civilian areas were not targeted.

The Britain-based Syrian Observatory for Human Rights said airstrikes in the central province of Hama on Thursday hit locations of the U.S.-backed rebel group, Tajamu Alezzah, as well as the province of Idlib, which is controlled by a coalition of rebel groups that include al-Qaida’s affiliate in Syria, Jabhat al-Nusra.

The British group said Tajamu Alezzah was also targeted on Wednesday.

Russia’s air campaign in support of Syrian government forces began Wednesday in what Putin called a pre-emptive strike against militants. Putin’s spokesman, Dmitry Peskov, said Russia was going after IS militants as well as a “list” of other groups.

“These organizations are well known and the targets are chosen in coordination with the armed forces of Syria,” he said Thursday, without specifying.

On Wednesday, however, Sergei Ivanov, Putin’s chief of staff, said “the operation’s target is solely air support for the Syrian government forces in their fight against the ISIS.”

Putin has said Russia would be fighting “gangs of international terrorists.” The Syrian government considers all rebel groups terrorists.

In Paris, Russian Ambassador Alexander Orlov insisted that Russian warplanes in Syria were hitting at the same extremists targeted by the U.S. and denied American claims that its military failed to coordinate the airstrikes.

Orlov said the targets were installations for IS and Jabhat al-Nusra, “two terrorist organizations recognized as such.”

The U.S. and Russia agree on the need to fight IS but not about what to do with President Bashar Assad. The Syrian civil war, which grew out of an uprising against Assad, has killed more than 250,000 people since March 2011 and sent millions of refugees fleeing to other countries in the Middle East and Europe.

With American and allied airstrikes daily, and now Russian warplanes in the Syrian airspace, the war is taking on a dangerous new dimension.

Orlov said Russian officials warned the Americans “via confidential channels” of where they planned to strike.