AMMAN/ANKARA (Reuters) - A Turkish army reconnaissance team scouted out Syria s Idlib province on Sunday, a senior Syrian rebel said, before an expected military operation to impose peace in the bitterly contested Syrian northwest.  When we don t go to Syria, Syria comes to us,  Turkish President Tayyip Erdogan told members of his ruling AK Party.  If we didn t take our measures, bombs would fall on our cities.  Turkey says it will provide assistance to rebels it has long backed, aiming to implement a de-escalation agreement designed to reduce fighting with pro-government forces in the area, the most populous pocket of Syria still in rebel hands. Although Turkish officials say they are helping the rebels from within Turkey, in past interventions they have also sent troops across the frontier. Any incursion could bring them into conflict with powerful jihadists from al Qaeda s former Syrian branch, although there were signs on Sunday that they were working to avoid a confrontation. Local sources said the Turkish military vehicles were escorted into Idlib by a convoy of fighters from the group, which has joined a jihadist alliance called Tahrir al-Sham and disavowed links to the network founded by Osama bin Laden. Nonetheless, the jihadists and the Turkish military earlier exchanged fire nearby, underscoring the tension as Turkey builds up its forces and the rebel groups it backs prepare to enter Idlib.   We will never allow a terror corridor that begins in Afrin and goes to the Mediterranean,  Erdogan said, referring to the stretch of Turkey s southern border which is controlled on the Syrian side by Kurdish fighters and Tahrir al-Sham. Turkey has been one of the biggest supporters of rebels fighting Syrian President Bashar al-Assad during the six-and-a-half-year war, but its focus has moved from ousting him to securing its own border against jihadist and Kurdish groups.     Prime Minister Binali Yildirim stressed the importance of ensuring de-escalation near the border, following an agreement reached in Astana, Kazakhstan last month that was backed both by the Turks and by Russia, which supports Assad.  We will ensure safety in Idlib, and will cooperate with Russia,  Yildirim said. Tahrir al-Sham, spearheaded by the former Nusra Front, which was al Qaeda s Syrian branch until last year when it changed its name and broke allegiance to the global network, has been a formidable military force since early in the conflict. Since early this year it has battled other rebel groups as it tried to gain control over areas including Idlib.  Idlib and neighboring parts of northwest Syria are now home to more than 2 million people, many of them refugees from other formerly rebel-held regions that fell to pro-government forces.      A local resident and another local rebel said they had seen Turkish military vehicles enter Idlib and then travel under Tahrir al-Sham escort along a road.  The senior Syrian rebel said the reconnaissance team went to Sheikh Barakat, a location that overlooks both rebel-held areas of Aleppo province, adjacent to Idlib, and the Kurdish-controlled area of Afrin.  Erdogan gave no details of the operation, but said it was going ahead  without problems at the moment . Turkey s biggest security concern on the frontier is the Kurdish YPG militia, which as part of a U.S.-backed alliance is fighting against Islamic State in eastern Syria. Ankara regards the YPG as an extension of the Kurdistan Workers Party (PKK), which has fought a three-decade insurgency inside Turkey. Mortars were fired from the area under YPG control near to the location where the Turkish reconnaissance team was operating, the senior rebel said.   The Turkish team is on its way back. Its mission has been accomplished. They visited areas of disengagement and locations where the Turkish army would be positioned,  the rebel said. Reuters witnesses, local people, and the Syrian Observatory for Human Rights, a Britain-based war monitor, said earlier on Sunday the Turkish military and Tahrir al-Sham had clashed near the village of Kafr Lusin in Idlib.  Tahrir al-Sham began the exchange by firing on a Turkish bulldozer removing sections of a border wall and Turkish artillery returned fire, they said. The area was later quiet.  Rebel groups that will take part - drawn from the Euphrates Shield campaign that Turkey backed with armor and troops in another part of Syria to the east since last year - said on Saturday they expected the Idlib operation to start very soon. Tahrir al-Sham said  the lions of jihad and martyrdom will be waiting to pounce  against any incursion into Idlib. The Syrian Civil Defence, better known as the White Helmets rescue workers, said on its Twitter account that jets it believed to be from the Syrian military had struck a marketplace in Maarat al-Numan in Idlib on Sunday, killing at least six.  