There have been reports of at least 36 civilian casualties.
Russian defence officials say their aircraft carried out about 20 missions targeting the self-proclaimed Islamic State, or IS.
But United States officials say there is no indication they were actually targeting IS-held territory.
Undeterred by smoke, several fires and the sounds of jets overhead, men ran into destroyed buildings searching for survivors.
Russian Defence Ministry spokesman Major General Igor Konashenkov says the country’s air force has targeted IS facilities.
He says they included military equipment, communication facilities, arms depots, ammunition and fuel supplies.
“Today, Russia’s air force carried out targeted strikes at eight objects of the Islamic State terrorist organisation on the territory Syrian Arab Republic. Around 20 plane flights have been carried out.”
But there are conflicting reports from United States officials and from activists within Syria.
US officials say the air strikes did not hit I-S targets.
US Defense Secretary Ash Carter says, despite Russian claims, it appears the air strikes took place in areas where there are probably no fighters from IS, also known as ISIL.
“I want to be careful about confirming information, but it does appear that they were in areas where there probably were not ISIL forces. And that is precisely one of the problems with this whole approach.”
Russian Foreign Minister Sergei Lavrov has dismissed claims by the US Pentagon on the Russian strikes.
“Everything was said by the Russian Minister of Defense. Don’t listen to the Pentagon about the Russian strikes.”
British foreign secretary Philip Hammond says Russia must confirm exactly what happened and that the targets were I-S and al-Qaeda, not President Bashar al-Assad’s foes.
“The targets of those strikes will not have been selected carelessly or at random. And it’s very important that Russia is able to confirm to the international community that the military action it has undertaken in Syria this morning is directed at ISIL and AQ-affiliated targets only — and not at moderate opponents of the Assad regime.”
The attacks were unleashed just hours after Russia’s parliament approved the use of force abroad.
Russian president Vladimir Putin says Russia will work with the Syrian army, in accordance with an official request by President Assad.
“First of all, we will support the Syrian army only in its legitimate fight, specifically against terrorist groups. Secondly, the support will be from the air without participation in ground operations. And, third, such support will be limited in time, as long as the Syrian army is on the offensive.”
The United States had welcomed Mr Putin’s stated purpose to help fight IS.
Earlier, at the United Nations General Assembly, where leaders talked over the conflict in Syria, US Secretary of State John Kerry warned Russia it would be closely watched.
“We must not, and will not, be confused in our fight against ISIL with support for Assad. Moreover, we have also made clear that we would have grave concerns should Russia strike areas where ISIL and al Qaeda-affiliated targets are not operating. Strikes of that kind would question Russia’s real intentions, fighting ISIL or protecting the Assad regime.”
Ash Carter, the US defence secretary, says Russia’s support of the Syrian leader poses a difficult problem.
“There is a logical contradiction in the Russian position, and now its actions, in Syria. Russia states an intent to fight ISIL on the one hand and to support Bashar al-Assad and his regime on the other. Fighting ISIL without pursuing a parallel political transition only risks escalating the civil war in Syria. So, this approach is tantamount to, as I said then, to pouring gasoline on the fire.”
Russia’s Defence Ministry did confirm the targets were based on information received from the Syrian army.
Mr Kerry says he and Mr Lavrov have spoken about the air strikes and the need to avoid a US-Russian conflict.
He says they are seeking clarity about Russia’s targets from US and Russian military officials.
“We agreed on the imperative of ‘as soon as possible,’ perhaps even as soon as tomorrow, but as soon as possible, having a military-to-military de-confliction discussion, whatever can be done as soon as possible, because we agree on the urgency of that de-confliction.”