NATO has told Russia to halt its incursions into the airspace of Turkey, a member of the alliance, saying the violations were both extremely dangerous and irresponsible.
Following an emergency meeting in Brussels, NATO ambassadors called on Russia to immediately explain itself, while expressing concern for Russian military attacks on Western-backed rebels in Syria.
"Allies strongly protest these violations of Turkish sovereign airspace, and condemn these incursions into and violations of NATO airspace," NATO said in a statement.
"Allies also note the extreme danger of such irresponsible behaviour. They call on the Russian Federation to cease and desist, and immediately explain these violations."
NATO said Russian military actions had reached a "more dangerous level" after two separate violations of Turkish airspace on Saturday and Sunday by Su-30 and Su-24 aircraft in the Hatay region on its southern border with Syria.
Turkey had only reported one violation by Russian aircraft, on Saturday.
"The aircraft in question entered Turkish airspace despite Turkish authorities' clear, timely and repeated warnings," the statement said.
"In accordance with NATO practice, Turkish fighter aircraft responded to these incursions by closing to identify the intruder, after which the Russian planes departed Turkish airspace."
NATO added: "The security of the Alliance is indivisible, and Allies stand in strong solidarity with Turkey," it said.
NATO has stationed Patriot missiles on Turkey's southern border with Syria to prevent any spillover from a bitter conflict which has left 250,000 people dead and sparked a mass exodus of migrants seeking safety in Europe.
The Patriots, which can shoot down aircraft as well as missiles, are however due to be pulled out shortly and it is uncertain if they will be replaced. NATO has said previously the issue was under review.
Russian strikes 'aimed at supporting Syrian army'
Russian military jets carried out strikes on nine so-called Islamic State sites in Syria over the past 24 hours, the defence ministry in Moscow has said.
The Russian warplanes hit IS command centres, weapon caches, artillery and communication posts in the Homs, Idlib, Hama and Latakia provinces of Syria, the ministry said in a statement.
Russia's operations in Syria are aimed at supporting the Syrian army and target terrorists and extremists there, Kremlin spokesman Dmitry Peskov has said.
"The aim of our operations lies in providing support to the Syrian military's offensive in their struggle with terrorist and radical organisations and forces," Mr Peskov said.
Meanwhile, US Defence Secretary Ash Carter has said that Russia is escalating Syria's civil war by targeting the moderate opposition.
He compared Moscow's effort to bolster Syrian President Bashar al-Assad to tethering itself to a sinking ship.
"By taking military action in Syria against moderate groups targets, Russia has escalated the civil war," Mr Carter said in a speech during a trip to Spain.
