Washington's refusal to jointly fight al-Qaida's branch in Syria has contributed to an escalation of fighting in the war-torn country, Russia's Defense Ministry said Friday.

The Russians proposed last week that Russia and the U.S.-led anti-Islamic State coalition launch joint action against the Nusra Front, but the U.S. military said its contacts with Russia are only to maintain airspace safety in the crowded skies over Syria.

Lt. Gen. Sergei Rudskoi of the Russian military's General Staff said the U.S. refusal to consider join action against the Nusra Front is leading "to further escalation of the military conflict."

He noted that the U.S. has failed to encourage opposition groups eager to abide by a U.S.- and Russian-brokered cease-fire in Syria to leave the areas where the al-Qaida affiliate is present, saying their failure to do so is threatening the truce.

A cease-fire that began Feb. 27 has helped reduce hostilities in some areas of Syria, but intense fighting has continued around the northern city of Aleppo and other areas.

Rudskoi said the situation in the provinces of Aleppo and Idlib has considerably worsened recently as "Nusra leaders sought to undermine the reconciliation process." He said the group has managed to replenish supplies and seize several villages, taking advantage of the fact that Russian warplanes haven't targeted areas where moderate opposition units are located close to Nusra positions.

Rudskoi said Nusra is now "the main obstacle to expanding the cease-fire regime in northern Syria."

"Further foot-dragging by our U.S. partners on the issue of separation of opposition units they control from terrorists don't only discredit the so-called 'moderate opposition,' but could lead to the collapse of the peace process and the resumption of fighting in Syria," he told reporters at a briefing.

Rudskoi also said the Russian military has intensified airstrikes on oil infrastructure and trucks used by Syrian militants to carry oil to Turkey since May 20.