Russia launched its first air strikes in war-torn Syria on Wednesday after President Vladimir Putin won parliamentary permission to use force abroad.
It was Moscow's first military engagement in a distant theatre of war since the Soviet invasion of Afghanistan in 1979.
Putin said the action was pre-emptive, warning that Moscow would be hunting down Islamic State militants before they target Russia.
"The only correct way to fight international terrorism... is to act preemptively, to battle and destroy fighters and terrorists on the territories they have already seized, not to wait for them to come to us," Putin said in televised comments.
The Russian defence ministry said Moscow had launched targeted air raids against the "terrorists" in Syria.
A Syrian security source said the war planes hit three provinces.
The strikes came as Putin and US President Barack Obama push rival plans on ways to defeat the Islamic State group in Syria and on the future role of the country's embattled leader Bashar al-Assad.
"They gave us a heads-up they were going to start striking in Syria," a US defence official said. "It was in the vicinity of Homs."
Putin had just hours earlier won permission from the upper house of parliament, the Federation Council, to deploy troops abroad.
"The Federation Council unanimously supported the president's request," Kremlin chief of staff Sergei Ivanov said.
He said the decision was taken after Assad asked Russia for military support, a request confirmed by Damascus.
Ivanov, a former defence minister, declined to give details of the operation, saying only it would be limited in duration and ruling out ground operations by Russian troops.
Putin is seeking to muscle his way back onto the world stage after months of Western isolation following Russia's seizure of Crimea from Ukraine and support for a separatist insurgency in the east of the ex-Soviet country.
- Differences with the West -
Russia will later Wednesday preside over a special UN Security Council meeting on countering terrorist threats that is bound to produce a sharp difference of views between Moscow and Washington.
Addressing the UN General Assembly for the first time in a decade, Putin on Monday proposed a broad UN-backed coalition to fight IS militants and clashed with Obama on the future of Assad.
Washington and its allies blame Assad for the mayhem in Syria, where four years of bloodshed have killed more than 240,000 people.
France meanwhile said it has launched a probe into Assad's regime for alleged crimes against humanity, saying it was forced to act in the face of "systematic cruelty".
The French investigation is largely based on evidence from a former Syrian army photographer known by the codename "Caesar," who defected and fled the country in 2013, bringing with him some 55,000 graphic photographs.
France, which is part of the US-led coalition against IS, carried out its own first air strikes against extremist positions in Syria on Sunday.
The Syrian Observatory for Human Rights monitoring group said the bombs killed at least 30 jihadists, including 12 child soldiers.
Russia argues that the West should support Assad in his fight against the jihadists.
But Washington says the Syrian leader must go if the Islamic State group is to be defeated.
- National interests, not ambitions -
The Pentagon says Russia has in recent weeks sent warplanes and other military hardware to northwestern Syria -- along with at least 500 troops -- in what many fear is an attempt to keep the country's president in power.
Moscow's proposal has exposed differences among Washington's European allies, with some siding with Obama and others saying Moscow should have a greater role in fighting IS.
Putin had also sought permission from the council to deploy military forces in Ukraine ahead of Russia's annexation of Crimea from Ukraine in March 2014.
Konstantin Kosachev, head of the council's international affairs committee, said Russia's forces would work in close coordination with Syria's army and stressed Moscow would not be sucked into a protracted conflict.
Ivanov said Russia was acting in the national interest to prevent foreign terrorist fighters from crossing its borders.
"We are not talking about achieving some foreign policy goals, satisfying someone's ambitions -- what our Western partners regularly accuse us of."
- 'Hide your sons' -
But many analysts accused the Kremlin of a short-sighted approach.
Alexander Konovalov, head of the Strategic Analysis Institute, said Russia was guided by a desire to end its diplomatic isolation and may not fully realise long-term consequences of a military involvement in Syria.
"We were coming to Afghanistan for six months and stayed there for 10 years," he told AFP, referring to a conflict that killed over 14,000 Soviet troops between 1979 and 1989.
According to a recent poll by the respected Levada Centre, 69 percent of Russians are against Moscow's deployment of troops in Syria, with just 14 percent in favour.
Wednesday's news set social networks alight, with many commentators predicting dire consequences for Russians.
"Hide your sons," one Russian, Zaira Abdullaeva, wrote on Facebook.
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