MOSCOW (Reuters) - Russia said on Friday it had intensified air strikes against oil sites controlled by an al Qaeda affiliate in Syria, but criticized the United States for refusing to join in.

Last Friday Russia proposed to the United States and its allies that they stage joint air strikes on Syrian rebels, including the militant Islamist Nusra Front, who are not observing a ceasefire, but Washington made clear it had little interest in the idea.

“The response received from the United States ... does not envisage joint actions against terrorist organizations, which leads to further escalation of the conflict,” Sergei Rudskoy, head of the General Staff’s main operations command, told a news briefing.

Meanwhile, the Nusra Front has partially restored its fighting efficiency, replenished stocks of weapons and ammunition and begun active military actions, Rudskoy said.

He said it was taking advantage of a previously announced cessation of hostilities in many locations, and of the fact that its units are often deployed in the same areas as the moderate opposition.

“Unfortunately, our American partners are not taking any decisive steps apart from persistent requests not to strike the groups of the Nusra Front, because ‘moderate opposition’ units may be located nearby,” Rudskoy said.

After discussing with U.S. experts the need to undermine the economic potential of the jihadists, Russian planes intensified strikes from May 20 against Nusra’s oil production sites and smuggling routes to Turkey, Rudskoy said.

But the key question remains unsolved, he said.

“Further delays by our American partners in resolving the issue of differentiating the opposition units it controls from terrorists ... leads to the disruption of the peace process and resumption of military actions in Syria.”

Washington has consistently refused to join forces with Russia in Syria ever since Moscow launched its campaign of air strikes in September last year, accusing it of acting solely to prop up Syrian President Bashar al-Assad.