(CNN) Turkey has only just begun to feel repercussions for downing a Russian warplane last week, Russian President Vladimir Putin warned during his annual state of the nation address Thursday, hours before the countries' foreign ministers were to meet amid simmering tensions.

"If somebody thought that after committing a treacherous war crime -- the killing of our people -- it would be possible to get away with mere restrictions on the trade of tomatoes, or some other restrictions ... then they are grossly mistaken," Putin said.

"We shall remind them many times about what they've done, and they will regret what they've done for a long time," he said. "We know what needs to be done."

Diplomatic salvos

Putin's tough talk was the just latest broadside in the feud that has erupted between the countries and their strongmen rulers since Turkey shot down a Russian warplane near the Turkey-Syria border November 24.

Ankara said the plane moved into Turkish airspace and received ample warning; Moscow denies both assertions, saying the aircraft was downed over Syria.

One of the plane's pilots was killed, while another was rescued.

Tensions between the countries have flared since the downing, with Putin describing the act as "a stab in the back by the terrorists' accomplices."

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Meanwhile, Turkish President Recep Tayyip Erdogan said he was "saddened" by the incident but has refused to apologize -- insisting that "those who violated our airspace are the ones who need to apologize."

In response, Russia has struck back with a raft of economic measures against Turkey, including suspending construction of a $12 billion pipeline intended to carry Russian gas to Turkey and to other European countries

Russian Foreign Minister Sergey Lavrov and Turkish Foreign Minister Mevlut Cavusoglu were expected to meet briefly Thursday afternoon in Belgrade, Serbia, on the sidelines of an Organization for Security and Co-operation meeting, the Russian state-run news agency RIA Novosti reported.

ISIS oil-buying allegations

Russia also has launched accusations that Turkey is involved in a secret oil trade with ISIS , the Islamist terror group that has captured parts of Syria and Iraq.

On Wednesday, Russian military officials outlined what they said was "hard evidence" of Turkish involvement in the trade -- alleging Erdogan and his family were involved in the business.

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Erdogan strongly denied the allegations, telling an audience Wednesday at Qatar University that "no one has the right to slander Turkey, especially the slander of Turkey buying ISIS oil. ... Turkey has not lost its moral values to buy oil from a terror organization."

Rather, Turkey's semiofficial Anadolu Agency ran reports that Syria's ruling Assad regime -- which Russia backs in the country's civil war -- was buying oil from the terror group.

Syrian leader speaks up

Syrian President Bashar al-Assad has waded into the diplomatic spat between one of his closest backers and his hostile neighbor, opining Sunday to a reporter from Czech Television that Turkey's downing of the plane "has shown the real intention of Erdogan, who, let's say, lost his nerve just because the Russian intervention has changed the balance on the ground."

"So the failure of Erdogan in Syria, the failure of his terrorist groups, means his political demise," he told the reporter. "So he wants to do anything in order to put obstacles in front of any success."

Syria's government has credited Russia for making major inroads against "terrorists" through its air campaign in the country. But critics have questioned whether Moscow is adequately prioritizing ISIS targets, as opposed to lashing out at those opposing Assad's regime, including more moderate factions.

In his speech Thursday, Putin offered thinly veiled criticism of recent Western policy toward the Middle East, saying that "previously successful and stable countries of the Middle East and North Africa -- Syria, Libya, Iraq -- have become a zone of anarchy and chaos posing a threat to the whole world."

He continued: "We know why it happened and who is behind the toppling of undesirable regimes ... thereby opening a way to radicals, extremists and terrorists."