UNITED NATIONS — For the second time this month, Russia moved to expand its political and military influence in the Syria conflict and left the United States scrambling, this time by reaching an understanding, announced on Sunday, with Iraq, Syria and Iran to share intelligence about the Islamic State.

Like Russia’s earlier move to bolster the government of President Bashar al-Assad by deploying warplanes and tanks to a base near Latakia, Syria, the intelligence-sharing arrangement was sealed without notice to the United States. American officials knew that a group of Russian military officers were in Baghdad, but they were clearly surprised when the Iraqi military’s Joint Operations Command announced the intelligence sharing accord on Sunday.

It was another sign that President Vladimir V. Putin of Russia was moving ahead with a sharply different tack from that of the Obama administration in battling the Islamic State, also known as ISIS or ISIL, by assembling a rival coalition that includes Iran and the Syrian government.

The effort, which Mr. Putin is expected to underscore in his speech at the United Nations on Monday, not only puts Moscow in a position to give military support to Mr. Assad, its longtime ally in the Middle East, but could also enable the Kremlin to influence the choice of a successor if Mr. Assad were to eventually leave power.