BEIRUT, LEBANON - Turkey escalated its confrontation with Syria on Wednesday, forcing a Syrian passenger plane to land in Ankara on suspicion of carrying military cargo, ordering Turkish civilian airplanes to avoid Syria's airspace and warning of increasingly forceful responses if Syria keeps firing artillery shells across the border.

NTV television in Turkey said two Turkish F-16 warplanes had intercepted a Syrian Air jetliner, an Airbus A320, with 35 passengers en route from Moscow to Damascus and had forced it to land at Esenboga Airport in Ankara, the capital, because it might have been carrying a weapons shipment to the Syrian government. Inspectors confiscated what NTV described as parts of a missile and allowed the plane to resume its trip after several hours. Turkish authorities declined to specify what had been found.

"There are items that are beyond the ones that are legitimate and required to be reported in civilian flights," Turkey's foreign minister, Ahmet Davutoglu, said in remarks reported by the country's semi-official Anatolian News Agency. "There are items that we would rate as troublesome."

There was no comment from Syria.

Turkish authorities said earlier in the day that all Turkish aircraft should avoid flying over Syria, possibly in anticipation of retaliation.

The steps taken by Turkey added ominous new tensions to its troubled relationship with Syria, where a nearly 19-month-old uprising against President Bashar Assad has evolved into a civil war and threatened to touch off a regional conflict. Turkey is the host for main elements of the anti-Assad insurgency and for 100,000 Syrian refugees, who have been fleeing in greater numbers as violence has increased along the 550-mile border in recent days. Several mortar rounds have landed on Turkish soil, prompting Turkish gunners to return fire.

News reports on Wednesday described intensified fighting close to Azamarin, a Syrian border settlement, with mortar and machine-gun fire clearly audible from the Turkish side. Wounded civilians, some of them in makeshift boats filled with women and children, could be seen crossing the Orontes River, which demarcates part of the Syrian border with Hatay Province in Turkey.

The Turkish chief of staff, Gen. Necdet Ozel, who visited parts of the border area on Wednesday, was quoted by Turkish news media as saying that military responses to Syrian shelling would be "even stronger" if the shelling persisted.

The rising tensions between Turkey and Syria are seen as especially troublesome because Turkey is a member of NATO, which considers an attack on one member an attack on all, and this implicitly raises the possibility that NATO will be drawn into a volatile Middle East conflict.

On Tuesday, the NATO secretary-general, Anders Fogh Rasmussen, emphasized that NATO had "all necessary plans in place to protect and defend Turkey if necessary."

The fighting in Syria has touched all other neighbors of Syria as well, with fighting reported recently in villages near a border crossing to Lebanon in the west, while in the east, Syrian authorities have lost control of some crossing points on the border with Iraq. Tens of thousands of Syrians have sought refuge in Lebanon and Jordan, straining resources in those countries. Last month, several mortar shells fired from Syria landed in the Golan Heights near Israel's northern border.