Istanbul, Turkey: Turkey's Recep Tayyip Erdogan warned Moscow against taking "undesirable steps" on Tuesday following accusations Russia deliberately violated Turkish air space in its bombing campaign in Syria.
Russian fighter jets entered Turkish air space in two separate incidents at the weekend, prompting Ankara to summon the Russian ambassador twice to protest both violations.
Meanwhile, the Turkish military has said its fighter jets were harassed by a MIG-29 plane from an unidentified country close to Syria on Sunday and Monday.
In his toughest remarks yet, the Turkish president lashed out at the incursions, saying "some undesirable steps have been taken and it does not befit Turkey to accept this."
"It is, of course, not possible to remain patient about this," he added on a state visit to Belgium.
Ankara's anger has intensified controversy over its bombing campaign in Syria which Turkey and the West fear is aimed at keeping President Bashar al-Assad in power.
Turkey and Russia have long been at loggerheads over the Syrian conflict, with Ankara seeking the overthrow of Assad while Moscow does everything to keep him in power.
"For us, this does not look like an accident, it is a serious violation," NATO Secretary General Jens Stoltenberg said in Brussels.
Russia has acknowledged one air incursion incident into Turkey on Saturday, which it blamed on "unfavourable weather conditions".
Russia and Turkey have in recent years sought to step up cooperation, agreeing to build a new gas pipeline beneath the Black Sea and Turkey's first nuclear power plant, while targeting $100 billion in bilateral trade from 2023.
But Erdogan warned that Russia risked losing Turkey as a friend if its behaviour continued.
"If Russia loses a friend like Turkey with whom it has a lot of cooperation it is going to lose a lot of things. It needs to know this," he added.