The main Syrian opposition council said Russia had stepped up air strikes since a U.S.-Russian ceasefire plan was announced on Monday and that it feared worse was to come in the days before the agreement is due to take effect on Saturday.
Salem al-Muslet, a spokesman for the High Negotiations Committee (HNC), also reiterated opposition fears that Russia will use the agreement to target Free Syrian Army rebel groups fighting to topple President Bashar al-Assad.
He said some terms of the deal indicated that it was heavily influenced by Russia and were obscure. "We fear that Russia will use this agreement to target the moderate factions in Syria," Muslet said in a telephone interview with Reuters.
The plan excludes militant groups such as Islamic State group and the al Qaeda-linked Nusra Front. But this, rebels say, will give the government a pretext to keep attacking them because militant combatants are widely positioned in opposition-held areas.
The Russian air force has been mounting air strikes in support of Assad since Sept. 30, shifting the momentum in his favour in a five-year-old conflict that has mostly reduced his control to the big cities of Syria's west and the coast.
IS group militants were reported to have tightened their grip on a Syrian government supply route to Aleppo on Tuesday as the army battled to retake the road as part of its campaign to seize the city.
As Damascus accepted a U.S.-Russian plan for a "cessation of hostilities" between the government and rebels due to take effect on Saturday, heavy Russian air strikes were also said to be targeting one of the last roads into opposition-held parts of Aleppo.
"The escalation in the bombing is targeting areas in Aleppo, in Homs and in Daraya," Muslet said, referring to the town of Daraya southwest of Damascus. "We expect more than that from the regime and from the Russian raids."
Muslet was speaking during a meeting of the Saudi-backed HNC in Riyadh. The HNC groups political and armed opponents of Assad.
The HNC said on Monday it "consented to" the international efforts but that acceptance of a truce hinged on an end to blockades of rebel-held areas, free access for humanitarian aid, release of detainees, and a halt to air strikes on civilians.
It also said it did not expect Assad, Russia, or Iran to cease hostilities.
"We are studying this truce and we are worried about the obscure points. There is no objection to the truce if it is implemented precisely, without Russia taking it as an excuse to target the moderate revolutionary factions," Muslet said.
Asaad al Zoubi, chief negotiator of the HNC, told pan-Arab Al Arabiya TV news channel that he expected the HNC meeting to spell out on Wednesday its concerns to Washington about the terms of the agreement before it came into effect.