Russian President Vladimir Putin expressed concern over the United States' refusal to share intelligence with Russia about terrorism in Syria on Tuesday, suggesting at an investment forum in Moscow that some American leaders have "mush for brains."

"I believe some of our partners simply have mush for brains," Putin said in what were some of his strongest comments yet about how the Obama administration is handling the Syrian crisis, reported AFP.

Russia's military entered the Syrian conflict late last month with the launch of a bombing campaign, saying it intended to take out the Islamic State group. Many of the attacks have been coordinated with Syrian President Bashar al-Assad, who the U.S. opposes and has been trying to overthrow for nearly a decade.

Washington is angry that Moscow also seems to be attacking anti-Assad rebels who are armed and funded by the U.S. Over the weekend, President Barack Obama accused Russia of intervening solely to prop up Assad's regime, according to CNBC.

But Putin denied that Russia was in Syria to back Assad, telling CNBC Tuesday that "we are not going to get into leadership (debates about Syria)."

"There is only one leader in Syria, which is the Syrian people. What we're trying to achieve is to contribute to the fight against terrorism, which is a threat both to the U.S., to Russia, to European countries and the whole world," he added.

Putin said that he often hears the U.S. complain "that our pilots are striking the wrong targets," and explained that Russia had asked Washington to provide it with a list of targets and specifically where it should strike, but said the U.S. did not cooperate.

"At the military level, we asked [the U.S.] to give us the information regarding the targets that they believe are 100 percent belonging to terrorists, and what we received as an answer was that they weren't going to do that. Then, the second question was asked - please tell us which targets should not be attacked by us. No answer received! What should we do then?" he said, according to CNBC.

"The first thing we hear right now is that they're not ready to cooperate with us and that we're attacking the wrong targets," he added. "I haven't been inventing things, [we asked them] to give us targets we should not pound and there was no response."

Russian Foreign Minister Sergei Lavrov also spoke out against the Pentagon's recent decision to airdrop 50 tons of ammunition to Syrian rebels, saying it would end up in the hands of the Islamic State group, reported The International Business Times. U.S. weapons previously supplied to various rebels groups, and even the Iraqi Army and Afghan military, have been turned over to extremist groups after the U.S.-backed groups defected or were overrun, according to NBC News.

"Frankly speaking, we have next to no doubts that at least the bulk of these weapons fall into the hands of terrorists," Lavrov said in the TV interview, according to the Russian news site Tass. "This fact causes concern in the United States as well, where people, the Congress begin to ask questions about previous attempts to support 'moderate opposition.'"