MOSCOW — Russia and the United States signed an agreement Tuesday that regulates all aircraft and drone flights over Syria, the defense departments of both countries announced.
At a Pentagon briefing, Peter Cook, the department's press secretary, said the agreement, called a memorandum of understanding, established safety protocols requiring both sides to maintain professional airmanship at all times, use specific communication frequencies and establish a communication line on the ground.
Anatoly I. Antonov, the Russian deputy defense minister, said in a statement, "The memorandum contains a set of rules and restrictions aimed at preventing incidents between the Russian and U.S. aviation." He did not go into details but said it had "important practical significance."
The text of the memo was not released. Cook said the Russians had requested that it not be shared. He declined to elaborate on the ground communication line, including where exactly it would be located, but described it as a backup resource to "have real-time conversations" in case something went wrong with the air communications.
He added that the agreement did not establish the sharing of intelligence or target information in Syria and that it did not "constitute U.S. cooperation or support for Russia's policy or actions in Syria."
"We continue to believe that Russia's strategy in Syria is counterproductive and their support for the Assad regime will only make Syria's civil war worse," said Cook, referring to the Syrian president, Bashar Assad.
Antonov suggested that more concrete steps needed to be hammered out, including round-the-clock communications channels that would be established between the two relevant military commands and cooperation on issues like joint assistance in "critical situations."
Russia's Defense Ministry spokesman, Maj. Gen. Igor Konashenkov, told reporters that an agreement was important because the number of aircraft aloft in recent days had increased noticeably, with as many as 30 combat aircraft sharing the skies over the same area simultaneously.
"There are military transport planes, combat aircraft and drones of various size, including attack drones," Konashenkov was quoted as saying by the Tass news agency.
Although Russia said it had deployed more than 50 aircraft to Syria to fight the Islamic State, the attacks that started on Sept. 30 have mostly targeted central areas controlled by groups opposed to the Syrian government other than the Islamic State militants.
One of the reasons that President Vladimir Putin was believed to be intent on interfering in Syria was to try to break Russia out of the diplomatic isolation imposed by the West, and the agreement Tuesday appeared to be a step in that direction.
The statement from Russia's Defense Ministry expressed disappointment that the agreement was limited to technical military matters.
"The Russian side was seeking a more substantial agreement," it said.
Moscow had proposed a number of specific measures to deepen Russian-U.S. military cooperation to counter the threat from international terrorism, the statement said.
Speaking to senior law enforcement commanders Tuesday, Putin reiterated that Russia had deployed its military in Syria to prevent the threat from the Islamic State from spreading beyond the Middle East.
"By creating a stronghold in Syria and a number of other Middle Eastern states, the terrorists have been making plans to expand their activities and destabilize entire regions," Putin said.
The Russian Defense Ministry said that the steps it had hoped for included asking the United States to provide information about targets in Syria. The Pentagon has declined to do that, with U.S. officials saying privately that they feared that Syria might target groups supported by the United States and that such cooperation might feed suspicions among opposition groups that Washington and Moscow were allied against them.
Russia was also looking for closer cooperation in cases where aircraft are shot down or are forced to making an emergency landing, the statement said, including joint search and rescue missions. The U.S. rejected the idea, according to the Russian Defense Ministry.
Finally, the Russian statement added the standard line that every official, starting with Putin, makes when discussing its role in Syria — that Russia has the legitimate right to be there because it was invited by Syria, whereas the U.S.-backed coalition is bombing Syria against international law.
The agreement Tuesday does nothing to change that, the Russian statement said.