EPA
Turkey has again summoned the Russian ambassador after a second violation of its airspace by a Russian warplane operating in Syria in two days.
It did the same after the first violation on Saturday, after which two Turkish F-16 jets were scrambled.
Turkey said the second violation occurred on Sunday.
Nato has urged Russia to end air strikes "on the Syrian opposition and civilians". Russia says it is targeting Islamic State and other Islamists.
Russia said Saturday's violation was for just a few seconds and due to poor weather. It has not officially reacted to Sunday's incident.
Saturday's interception took place near Yayladagi in the southern Hatay region, Turkey says.
A statement by Nato's 28 members, which include Turkey, warned of "the extreme danger of such irresponsible behaviour" and urged Russia "to cease and desist".
US Secretary of State John Kerry said Turkey would have been within its rights to shoot the jets down.
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"The Turkish Armed Forces are clearly instructed," Turkish Prime Minister Ahmet Davutoglu told Turkish TV. "Even if it is a flying bird, it will be intercepted."
But he played down the possibility of a "Turkey-Russia crisis", saying that channels between the two countries remained open.
UN spokesman Stephane Dujarric expressed concern at the dangers of having "a lot of different countries and different coalitions" carrying out air campaigns in Syria, saying the situation was "fraught with danger". He urged a renewed focus on a political situation to the conflict.
The Russian air campaign began on Wednesday, with Moscow saying it was targeting IS positions and those of other Islamist groups. Syria said on Monday that the air strikes had been planned for months.
The country's defence ministry said 10 targets had been hit during the course of 15 strikes on Monday. Among the targets Russia said it destroyed were:
- 20 tanks that had been seized from the Syrian army by militants
- two IS command centres, one near Damascus and one in Aleppo
- two ammunition dumps in Homs province
Mr Putin has denied that civilians have been killed in the past week, but evidence on the ground has indicated otherwise.
Turkey and other members of the US-led coalition in Syria say the principal target is in fact the Syrian opposition groups fighting President Assad.
Nato said Russian air strikes did not target IS positions, but said it should "focus its efforts" on doing so.
Earlier on Monday, Vladimir Komoyedov, the head of the Russian government's defence committee, said Russia had not ruled out attacking rebel positions using warships.
Analysis - Jonathan Marcus, BBC Defence & Diplomatic Correspondent
The Russian incursion into Turkish air space has set alarm bells ringing in Nato with Alliance ambassadors describing it as "irresponsible behaviour".
The US Secretary of State John Kerry has also expressed his concern, noting that this is "precisely the kind of thing" Washington warned about in pressing Russia for talks to avoid mid-air incidents.
Turkish jets shot down a Syrian Mig-fighter in March of last year. Another Syrian aircraft, possibly a helicopter, was shot down last May.
Nato has called on Russia to "take all necessary measures" to ensure that such violations do not take place in the future. American sources claim that, far from being an accident, the Russian pilots knew exactly what they were doing.
As the Russian air campaign moves into a higher gear, this episode illustrates some of the wider dangers involved.
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