Turkish President Recep Tayyip Erdogan warned Moscow on Saturday that it would be forced to "endure the consequences" if its jets continue to violate the Turkish airspace, after Ankara reported a new border infringement incident by a Russian plane.

NATO-member Turkey said another Russian warplane violated its airspace on Friday despite several warnings — two months after Turkey's military shot down a Russian jet for crossing over its territory. The past incident seriously strained the previously close ties between the two countries, damaging a strong economic partnership.

"We regard this infringement which came despite all our warnings in Russian and in English as an effort by Russia to escalate the crisis in the region," Erdogan told reporters before departing on a Latin American tour. "If Russia continues the violations of Turkey's sovereign rights, it will be forced to endure the consequences."

He did not specify what those consequences might be.

Erdogan said he attempted to reach Russian President Vladimir Putin to discuss the issue but that the Russian leader did not respond.

"These irresponsible steps do not help the Russian Federation, NATO-Russia relations or regional and global peace," Erdogan said. "On the contrary they are detrimental."

Turkey's foreign ministry said the Russian SU-34 crossed into Turkish airspace on Friday, ignoring several warnings in Russian and in English by Turkish radar units. It said Ankara summoned the Russian ambassador to the ministry Friday evening to "strongly protest" the violation. It was not clear where exactly the new infringement had occurred.

NATO urges 'calm and de-escalation'

NATO Secretary General Jens Stoltenberg also called on Russia "to act responsibly and to fully respect NATO airspace" but also urged "calm and de-escalation" of tensions between Moscow and Ankara.

"Russia must take all necessary measures to ensure that such violations do not happen again," he said in a statement. "NATO stands in solidarity with Turkey and supports the territorial integrity of our ally, Turkey."

There was no immediate comment on the incident from Moscow.

In November Turkey shot down a Russian plane which violated its airspace near Syria, touching off a crisis between the two countries. It was the first time in more than half a century that a NATO nation had shot down a Russian plane.

Turkey brought down the Russian Su-24 bomber near the border with Syria on Nov. 24, saying it violated its airspace for 17 seconds despite repeated warnings. Russia insists the plane never entered Turkish airspace. One pilot and a Russian marine of the rescue party were killed in the incident.

The Russian military quickly sent missile systems to Syria and warned that it would fend off any threat to its aircraft. Moscow also punished Turkey by imposing an array of economic sanctions.

On Saturday, Stoltenberg said NATO had agreed in December to increase the presence of AWACS early warning planes over Turkey to increase the country's air defences. He said the decision was taken before Friday's incident.

Russia denies jet was Russian

Russia's defence ministry denies that one of its jets violated Turkish airspace Saturday.

The ministry spokesman, Maj. Gen. Igor Konashenkov, said that "there were no violations of Turkish airspace by planes of the Russian air group in the Syrian Arab Republic. Turkey's statement on the alleged fact of airspace violation by a Russian Su-34 is unsubstantiated propaganda."

His comments were carried by state news agencies Tass and RIA Novosti.

He added that: "None of these radars are able to establish the type and the nationality of the aircraft — whether Russian or of the U.S.-led so-called antii-ISIS coalition."

Such identification "is possible only by means of direct visual contact with another aircraft, which there was not. To say seriously that with the help of these radars someone warned someone else 'in Russian and English' can only be done by ignorant propagandists watching Hollywood blockbusters."

Syrian opposition heads to Geneva for peace talks

Meanwhile, a delegation from Syria's main opposition group flew to Geneva on Saturday to assess whether to join Damascus government officials in United Nations-brokered peace talks, an opposition representative said.

The 17-strong team included the head of the Saudi-backed Higher Negotiation Committee (HNC), which includes political and militant opponents of Syrian President Bashar al-Assad in the country's five-year civil war.

A malnourished child lies in a bed waiting to receive treatment at a therapeutic feeding centre in a hospital in Sanaa, Yemen, on Sunday, Jan. 24, 2016. This child is one of millions of people across countries like Syria, Yemen and Iraq that are gripped by hunger, struggling to survive with little help from the outside world. (Hani Mohammed/Associated Press)

The HNC has said it wants to discuss humanitarian issues including a stop to Russian and Syrian government bombing before engaging in the peace talks that started on Friday in Geneva.

Russian air strikes on Syria have killed nearly 1,400 civilians since Moscow started its aerial campaign nearly four months ago, the Syrian Observatory for Human Rights, a monitoring group, said on Saturday.

"We are going to Geneva to put to the test the seriousness of the international community in its promises to the Syrian people and to also test the seriousness of the regime in implementing its humanitarian obligations," Riyad Naasan Agha said. "We want to show the world our seriousness in moving towards negotiations to find a political solution," he told Reuters.

French Foreign Minister Laurent Fabius said on Saturday the Geneva talks must ensure human rights are upheld as participants work towards a political transition in Syria. "Humanitarian law must be respected and the objective of a political transition actively pursued to enable the talks to succeed," Fabius said in a statement sent to Reuters.

Russian Deputy Foreign Minister Gennady Gatilov was quoted by Russian Interfax news agency as saying that no direct talks were expected in Geneva, only proxy talks.

Gatilov, whose country has also objected to the opposition's composition saying it included groups that it deemed as terrorist, said there were no preconditions for the Syrian talks and that Moscow welcomed the decision by Syrian opposition coordinator, Riad Hijab, to take part in talks in Geneva.

Rebel fighters of al-Jabha al-Shamiya (Levant Front) stand within the compound of the justice palace in the old city of Aleppo, Syria on Jan. 28, 2016. (Abdalrhman Ismail/Reuters)

Ceasefire sought

The UN earlier said the aim would be six months of talks, first seeking a ceasefire, later working toward a political settlement to a war that has killed more than 250,000 people, driven more than 10 million from their homes and drawn in global powers.

German Foreign Minister Frank-Walter Steinmeier told German newspaper Welt am Sonntag the negotiations would be a test of intentions.

"Only at the negotiating table will it become clear if both sides are prepared to make painful compromises so that the killing stops and Syrians have a chance of a better future in their own country."

The HNC's demands include allowing aid convoys into rebel-held besieged areas where tens of thousands are living in dire conditions, Agha said. The medical charity Medecins Sans Frontieres (MSF) said on Saturday that 16 people had starved to death in the government-besieged town of Madaya since aid convoys arrived this month and blamed the authorities for blocking medical supplies shipments.

"It is totally unacceptable that people continue to die from starvation, and that critical medical cases remain in the town when they should have been evacuated weeks ago," said Brice de le Vingne, MSF's director of operations in a statement.

Agha said the opposition delegation, including HNC head Hijab and chief negotiator Asaad al-Zoubi, would not call for a complete cessation of hostilities but would demand an end to "the indiscriminate shelling of markets, hospitals and schools by the regime and its Russian backers".

Russia and Syria deny targeting civilians, saying they take great care to avoid bombing residential areas.

Syrian refugee children play at Al Zaatari refugee camp in the Jordanian city of Mafraq, near the border with Syria on Jan. 30, 2016. (Muhammad Hamed/Reuters)

Russian bombing continues

Separately, the heavy Russian aerial bombing campaign continued unabated in northern Syria on Saturday with the Syrian Observatory for Human Rights saying the areas hit included rebel-held villages and towns in the Aleppo countryside near the border with Turkey.

Russian aerial strikes were also reported by the monitor in Hama countryside and in the eastern province of Deir al Zor.

Heavy clashes also continued in the Latakia countryside where the Syrian army backed by intensive Russian carpet bombing in the rugged mountainous area allowed the government to regain most of the countryside close to the coastal heartland of Assad's Alawite sect.