Russian President Vladimir Putin said Monday that Turkey shot down an Su-24 bomber, which Ankara insists violated its airspace, in order to protect supplies of oil for the "Islamic State" (IS).

"We have every reason to think that the decision to shoot down our plane was dictated by the desire to protect the oil supply lines to Turkish territory, right to the ports where it is loaded onto tankers," Putin told reporters in Paris on the first day of the UN climate conference.

Revenue from seized oil fields in Syria and Iraq is one of the main sources of income for IS.

Turkish President Recep Tayyip Erdogan responded by offering to resign if the allegations could be proved.

"If such a thing is proved, the nobility of our nation would require that I would not stay in office," Erdogan said in remarks carried by the state-run Anadolu news agency.

The Russian leader has rebuffed Turkey's invitations to meet on the sidelines of the climate conference in France.

Deteriorating relations over downed jet

The downing of the Russian jet on the Turkey-Syria border has strained ties between the two countries, which have had a close economic relationship despite differences over Syria.

Turkish Prime Minister Ahmet Davutoglu rejected calls to apologize over the incident.

"Protection of our airspace, our border, is not only a right but a duty for my government. No Turkish premier or president ... will apologize (for) doing our duty," Davutoglu told a joint press conference with NATO head Jens Stoltenberg in Brussels.

Russia has already announced retaliatory economic measures designed to hurt the Turkish economy, especially the key tourism and agricultural sectors. Moscow will halt fruit and vegetable imports from Turkey after Putin signed a decree over the weekend that also bans charter flights and the sale of package holidays. Russia has reportedly scrapped its visa-free regime for Turkish nationals.

Prime Minister Dmitry Medvedev called the moves a "first step" as Moscow also said it would limit Turkish transport firms and tighten controls on construction contracts, though the moves have so far not touched Russia's major energy projects.

Davutoglu criticized Moscow's decision, but he remained conciliatory in his words.

"Economic relations are based on mutual interest, and we will approach any other measures very calmly," Davutoglu said. "We will not escalate the tensions. We will never do anything that would be seen negatively by the Russian people."

jar/gsw (AFP, dpa, AP)