NATO on Monday condemned Russian incursions into Turkish airspace as an "extreme danger" and demanded that Moscow halt all attacks against the Syrian opposition and civilians.
"Allies strongly protest these violations of Turkish sovereign airspace and condemn these incursions into and violations of NATO airspace," the US-led alliance said after an emergency meeting on the Syria crisis at its Brussels headquarters.
"Allies also note the extreme danger of such irresponsible behaviour," it said in a statement.
Ambassadors from the 28 NATO member states said they were deeply concerned by Russia's military build-up in Syria, and by attacks which caused civilian casualties and did not hit Islamic State jihadists (ISIL), as claimed by Moscow.
"Allies call on the Russian Federation to immediately cease its attacks on the Syrian opposition and civilians, to focus its efforts on fighting ISIL, and to promote a solution to the conflict through a political transition," it said.
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.
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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."
Russia must "cease and desist and immediately explain these violations," 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.