ANKARA/ALEPPO

Regime forces backed by Russian airpower have continued to strike opposition positions in Syria despite a cessation of hostilities agreement -- unveiled last week by Moscow and Washington -- that went into effect midnight Friday.

Over the last three days, Russian warplanes have targeted villages in the west central Hama province and the northwestern Aleppo province, according to local sources.

At least 12 civilians were killed and 12 others injured Sunday by Russian airstrikes carried out in northern and western Aleppo and southern Hama -- all regions held by the Free Syrian Army.

But in Washington, the U.S., which jointly with Russia co-chairs the International Syria Support Group (ISSG), said it was too early to designate blame for any potential violations.

"We're aware that there have been some reports of violations, some concerns that have been raised," White House spokesman Josh Earnest told reporters, noting that Washington expected violation heading into the deal's implementation.

"At this point, it's too early to say" where responsibility lies, he said.

Asked if it is too early to say if Russia in particular has conducted any violations, he said, "That is correct."

Despite an overall reduction in violence, there is still much-needed improvement to humanitarian access in the country, Earnest's State Department counterpart, John Kirby added.

Last week, the U.S. and Russia announced the cessation of hostilities agreement.

The Daesh terrorist group and the Nusra Front, Al-Qaeda’s Syrian affiliate, are excluded from the agreement. On Friday, Russian President Vladimir Putin said Moscow would continue military operations against Daesh and other terrorist groups operating in Syria "even after the cease-fire takes effect".

But while Moscow claims to be confining its airstrikes in Syria to Daesh positions, some members of the western NATO alliance say it is targeting moderate opposition groups opposed to the Assad regime.

Since last September, ongoing Russian airstrikes have forced more than half a million Syrians to flee their homes in the war-torn country, with many seeking shelter in Turkey or other neighboring states.

Since the truce went into effect, the Assad regime -- which along with Russia is also supported by Lebanon’s Hezbollah group -- has also continued to carry out attacks, both from the air and on the ground.

These have occurred in the Homs, Aleppo, Hama and Damascus provinces, as well as the Turkmen Mountain region in the province of Latakia. Riad Hijab, general coordinator of Syria’s pro-opposition High Negotiations Committee, said in a letter to the UN that at least 29 people had been killed by regime attacks since the truce went into effect.

The U.S., EU, Turkey and the Arab Gulf States, for their part, say that only when Russia halts airstrikes against opposition-held and civilian areas in Syria can a genuine cease-fire be achieved.