MOSCOW — Russia on Monday condemned the American military’s downing of a Syrian warplane, suspending the use of a military hotline that Washington and Moscow have used to avoid collisions in Syrian airspace and threatening to target aircraft flown by the United States and its allies over Syria.
The moves were the most recent example of an intensifying clash of words and interests between the two powers, which support different sides in the yearslong war in Syria.
The Russian military has threatened to halt its use of the hotline in the past — notably after President Trump ordered the launch of missiles against a Syrian air base in April — only to continue and even expand its contacts with the United States military. It was not clear whether the latest suspension would be lasting.
Its announcement came in response to an American F/A-18 jet’s shooting down a Syrian government warplane south of the town of Tabqah on Sunday, after the Syrian aircraft dropped bombs near local ground forces supported by the United States. It was the first time the American military had downed a Syrian plane since the civil war began in the country in 2011.
The Russian Defense Ministry called American attacks against the Syrian forces “military aggression” and announced that it would suspend cooperation with the United States intended to prevent airborne accidents over Syria.
“All flying objects, including planes and drones of the international coalition, detected west of the Euphrates, will be followed by Russian air defense systems as targets,” the Russian Defense Ministry said in a statement.
The United States will continue to conduct air operations over Syria, a spokesman for the American-led task force that is fighting the Islamic State said on Monday.
“We are going to continue to conduct operations throughout Syria, providing air support for coalition and partnered forces on the ground,” the spokesman, Col. Ryan Dillon, said in a telephone interview from Baghdad.
Colonel Dillon did not provide details on what air operations were underway. After the United States carried out the cruise missile attack in April at a Syrian airfield that was used to mount a nerve-gas attack, the American-led air war command initially sent armed drones in and around Raqqa instead of piloted aircraft. That was done to guard against the risk of retaliation by Syrian and Russian air defenses.
Weeks after President Vladimir V. Putin of Russia ordered his country’s military forces to Syria in September 2015 to prop up the government of President Bashar al-Assad, Russia and the United States signed amemorandum on preventing air clashes between the two countries.
Since then, the agreement has been a crucial link that has allowed Moscow and Washington to notify each other about their air operations over Syria, in which Iran, Israel, Russia, Syria, Turkey and the United States with its allies carry out attacks in pursuit of often-competing aims.
But Moscow has tried to use the agreement as a leverage each time the situation has threatened to escalate.
The Russians threatened to stop using the hotline after the April cruise missile strike. But by the next month, the two sides were using it more than ever.
Colonel Dillon said the American-led coalition was also prepared to continue using the hotline, which consists of phone calls between the United States’ Al Udeid Air Base in Qatar and a Russian base at Latakia, Syria. An unclassified Gmail account is used as a backup.
“The coalition is always available to deconflict with Russia to ensure the safety of our aircrews and operations,” Colonel Dillon said.
American officials, who declined to be identified because they were discussing internal procedures, said that the two sides had used the hotline on Sunday.
Speaking in Beijing on Monday, the Russian foreign minister, Sergey V. Lavrov, seemed to be unaware of the decision. He called on the United States and all other countries involved in the Syria conflict to “coordinate their actions.”
“We urge everyone to avoid acting unilaterally, to respect the sovereignty of Syria,” he said.
In Moscow, Frants Klintsevich, the deputy chairman of the Russian Senate’s defense committee, called the downing of the Syrian warplane a “blunt act of aggression and provocation.”