By Dominic Evans
ISTANBUL (Reuters) - Strains between Turkey and Russia over the escalating Syrian offensive in Idlib appear not to have shaken Turkish plans to deploy Russian missile defence systems, a senior U.S. official said on Friday, despite a threat of U.S sanctions.
The fighting in the northwestern Syrian province has led to testy exchanges between Russia, which supports the offensive by Syrian troops, and Turkey which has deployed thousands of its own soldiers to support insurgents trying to halt the advance.
Last week President Tayyip Erdogan angrily told Russia to "step aside" and let Turkey retaliate after a number of Turkish troops were killed. Both countries have accused each other of flouting accords aimed at containing the conflict.
The falling-out has offered the United States a rare opportunity to highlight its common ground with NATO partner Turkey, after years of worsening relations which could deteriorate further if Turkey goes ahead with plans to activate the Russian S-400 missile system in coming months.
Secretary of State Mike Pompeo tweeted support for Turkey. Special envoy James Jeffrey, speaking in Turkish in the capital Ankara, referred to the fallen Turkish soldiers as "martyrs".
Turkey was defending its interests in Idlib "in a manner that is appropriate and one that the United States supports," the senior U.S. State Department official, who spoke on condition he was not further identified, told reporters.
Washington has also sought to emphasise Moscow and Ankara's divergent goals - not just in Syria but also in the Libyan conflict, where the two countries also support opposing sides.
"What we are seeing in Syria and Libya in particular is demonstrative of the way in which Turkish and Russian interests do not overlap," he said. "I'm hopeful that our Turkish partners will take that message away from this."
TURKEY'S COURSE "IS SET"
The official, speaking after talks in Turkey, said Russia's "destructive role" was having an impact on Turkish authorities, but had not led to a change of policy on the S-400s.
"I have not seen that translate into a rethink of Turkey's position specific to the S-400s. It is my expectation that Turkey's course on this is set," he said.
Turkey took delivery of a first batch of the S-400 missile defence batteries last July and has said it plans to activate them in April. Washington says they are incompatible with NATO defences and has halted Turkey's purchase of F-35 stealth fighter jets, which it says would be compromised by the S-400s.
The dispute, which could trigger U.S. sanctions on Turkey, is one of several disagreements which have soured relations including a U.S. conviction of a Turkish bank executive for evading U.S. sanctions on Iran and Washington's support for Kurdish YPG fighters in northeast Syria.
Until the latest escalation of fighting in Idlib, Turkish and Russian troops had carried out regular joint patrols in northeast Syria as part of an accord between Ankara and Moscow under which YPG fighters withdrew from the border.
A Turkish security source said a planned patrol last week had been postponed because of "harsh weather", but the U.S. official said the Idlib dispute could be having an impact.
"I assess that that is spilling over into Turkish-Russian cooperation in the northeast as well. The one has to complicate the other. You cannot isolate them."
(Reporting by Dominic Evans; Editing by Mark Heinrich)