NATO's secretary general on Tuesday accused Russia of deliberately violating Turkish air space in its bombing campaign in Syria, as Turkey's Recep Tayyip Erdogan warned Moscow against continuing such "undesirable" behaviour.

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.

The anger over Russia's air incursions has intensified controversy over its bombing campaign in Syria which Turkey and the West fears is aimed at keeping President Bashar al-Assad in power.

"For us, this does not look like an accident, it is a serious violation," NATO Secretary General Jens Stoltenberg said in Brussels.

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.

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.

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.

- Fresh raids on IS -

As diplomatic tensions simmered, the defence ministry in Moscow said Turkish officials had told the Russian military attache that they wanted to discuss setting up a working group between the two sides to avoid future incidents.

There was no immediate comment from officials in Ankara.

Meanwhile, Russian jets carried out air strikes on 12 Islamic State sites in Syria on Tuesday, as Moscow expanded its week-old bombing campaign in the war-torn country.

In a statement, the defence ministry said Russian warplanes had carried out some 20 sorties, hitting "logistical infrastructure, command posts, training camps and bases" belonging to IS jihadists around the eastern city of Deir Ezzor and in the provinces of Damascus, Idlib and Latakia.

The statement made no mention of a bombing near the city of Palmyra despite Syrian state TV reporting Russian bombing raids in and around the city. The defence ministry dismissed the report as "absolute lies".

On Sunday, jihadists had blown up Palmyra's Arch of Triumph, prompting fresh outrage at the destruction of the UNESCO World Heritage site.

Russia says it is striking against Islamic State (IS) jihadists and other terrorist groups in Syria after a conspicuous military-build up in the last weeks that raised international concern.

But the West and Turkey have accused Russia of striking at US-backed moderate rebels who have been working to overthrow Assad.

US Defence Secretary Ashton Carter expressed disappointment over the lack of new talks with Russia aimed at preventing accidents as the US-led coalition also bombs IS targets.

But on Tuesday, a senior Pentagon official said Russia had informally pledged to push ahead with efforts to avert potential mishaps with US pilots overflying Syria.

During talks last week, both sides agreed to a series of commitments, the source said, indicating it involved undertakings on which language pilots will use to communicate, the choice of radio frequency and the altitude at which they will operate.

The Pentagon acknowledged the move and said it hoped to hear a formal response from Russia.

The Russian operations come more than a year after a US-led coalition began bombing IS in Syria in a bid to destroy the group.

The advance of the extremists across Syria added to the exodus of refugees into neighbouring countries, including Turkey which is now hosting almost two million Syrians.

The EU said it would step up resettling refugees now in Turkey and help reinforce Turkish coastguard patrols under a crisis plan discussed during talks with Erdogan on Monday.

- 'Unfavourable weather' -

Russia has acknowledged one air incursion incident into Turkey on Saturday, which it blamed on "unfavourable weather conditions".

"There is no need to look for some conspiracy theories," the defence ministry said of the incident, which it added lasted "several seconds".

According to NATO, Turkish airspace was violated by Russian SU-30 and SU-24 aircraft in the southern Hatay region on Saturday and Sunday.

Meanwhile, the Turkish military said its F-16 jets were harassed and put on radar lock by an unidentified MIG-29 aircraft on the Syrian border Monday.

But it was unclear if the incident -- the second of its kind in two days -- was linked to Russia.

Meanwhile the Kremlin denied suggestions Russia is planning to send ground troops to Syria and said it will not support Russian volunteers willing to take part in the conflict.

bur-sah-sjw/hmw

Heritage