WASHINGTON — The already fragmented battlefield in Syria grew even more complicated on Friday, as Russia and Iran expanded their military efforts to defend the beleaguered Syrian government in defiance of President Obama, who predicted that their actions would lead only to a “quagmire.”

In his first comments since Russia began airstrikes on Syrian targets this week, Mr. Obama said that Moscow was acting “not out of strength but out of weakness.” Bristling at criticism of his own Syria policy, he rejected domestic opponents who offer “half-baked ideas” that amount to “a bunch of mumbo-jumbo.”

“An attempt by Russia and Iran to prop up Assad and try to pacify the population is just going to get them stuck in a quagmire and it won’t work,” Mr. Obama said during a news conference at the White House on Friday, referring to President Bashar al-Assad of Syria, a longtime ally of both Russia and Iran. “And they will be there for a while if they don’t take a different course.”

Neither Russia nor Iran showed signs of listening. While Moscow widened its airstrikes to hit Islamic State territory for the first time, Russian troops have unloaded a major long-range artillery system to add more firepower to its deployment in Syria, according to an American official. At the same time, American officials said Iran had sent additional ground troops to bolster Mr. Assad’s government.