Russian vehicles block a road leading to the ancient city of Palmyra in central Homs province, Syria, in April. (Hassan Ammar/AP)

Russia on Friday proposed conducting joint airstrikes with the U.S.-led coalition in Syria against an al-Qaeda-linked group and other factions, an escalation in an ongoing strategy by Moscow to seek more coordination with the West and its allies in the Syrian conflict.

Speaking at a gathering of military and political leaders in Moscow, Russian Defense Minister Sergei Shoigu suggested that joint airstrikes begin Wednesday against forces that have not signed a fast-unraveling truce in Syria’s civil war.

“We propose . . . a joint action between the Russian air force and the U.S.-led coalition to plan and conduct strikes against the al-Nusra Front, which does not support the cease-fire, as well as against convoys of arms and fighters crossing the Syrian-Turkish border,” Shoigu said, according to a translation of the televised remarks by Russian state media.

His reference was to Jabhat al-Nusra, often described as al-Qaeda’s affiliate in Syria.

The State Department responded quickly that such an agreement does not exist and that officials have only been discussing a better way to monitor and enforce a cease-fire in the country.

“There is no agreement to conduct joint airstrikes with the Russians in Syria,” said State Department spokesman John Kirby.

Several Pentagon officials also said they were not immediately aware of any changes in the arrangement for separate U.S. and Russian air operations in Syria. The officials spoke on the condition of anonymity because they were not authorized to brief reporters.

The Pentagon now holds periodic video conferences with Russian officials on their separate airstrikes in Syria but has said repeatedly that the communications are limited to flight safety. “We don’t have any plans to expand that,” one official said.

Suggestions of military cooperation between the United States and Russia against the Islamic State have circulated repeatedly since the two governments agreed early this year to head a diplomatic committee pushing for a political solution to the civil war, in which they support opposing sides.

In March, Russia announced that they were discussing “concrete” coordination to liberate the Islamic State stronghold of Raqqa. In January, Russian Foreign Minister Sergei Lavrov, who co-chairs the diplomatic initiative with Secretary of State John F. Kerry, accused the United States of refusing to hold “serious, adult-like” talks about Russian proposals to coordinate their military actions.

Senior Russian and U.S. military officers also meet regularly in Geneva under a new plan to monitor violations of a cease-fire begun in February under the Kerry-Lavrov initiative.

The truce has fallen apart in recent weeks as Syrian government forces, backed by Russia, attacked what they said were terrorists belonging to Jabhat al-Nusra. Those forces are interwoven with some rebel groups that are party to the cease-fire, and the United States has criticized Moscow and Damascus for using the Jabhat al-Nusra presence as an excuse to continue attacks on the opposition. Russia also accuses Turkey of providing support for terrorists.”

Russia, which had scaled back its airstrikes in support of the government when the truce began, more recently appears to have ramped them up again.

Shoigu indicated that Moscow was prepared to further increase its attacks. Moscow, he said, would “reserve the right to ­unilaterally conduct airstrikes against forces of the international terrorist organizations and militant groups that did not join the truce.”

Reports of increased Russian- U.S. military cooperation in Syria have unsettled the U.S.-backed opposition, which has said it will not continue peace talks with the government until the violence abates. Opposition spokesmen have expressed concern that the United States, whose primary interest is in the separate war against the Islamic State, will waver in its insistence that the internal Syrian conflict cannot end until Russian-backed President Bashar al-Assad leaves office.

The United States and NATO officially ceased all formal military cooperation with Russia following its 2014 annexation of Crimea, in Ukraine. NATO has also bolstered its forces in parts of Eastern Europe amid worries about Russian aggression.

Carol Morello in Brussels and Missy Ryan and Karen DeYoung in Washington contributed to this report.