BEIRUT, Lebanon -- Turkey sharply 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 stay out of Syrian airspace and warning of increasingly forceful responses if Syrian artillery gunners keep lobbing shells across the border.
Turkey's NTV television said two Turkish F-16 warplanes were dispatched to intercept a Syrian Air A-320 Airbus jetliner with 35 passengers en route from Moscow to Damascus, and forced it to land at Esenboga Airport in Ankara, because it may have been carrying a weapons shipment to the Syrian government. Inspectors confiscated what NTV described as parts of missile and allowed the plane to resume its trip after several hours. The Turkish authorities declined to specify what precisely had been found.
"There are items that are beyond the ones that are legitimate and required to be reported in civilian flights," Foreign Minister Ahmed Davutoglu of Turkey said in remarks reported by the country's semiofficial Anatolia News Agency. "There are items that we would rate as troublesome."
There was no immediate comment from the Syrian side. Turkish transportation authorities said earlier in the day that all Turkish aircraft should avoid flying over Syrian territory, possibly in anticipation of retaliatory action by Syria.
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 al-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 roughly 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 bombs have landed on Turkish soil, prompting Turkish gunners to return fire.
News reports on Wednesday spoke of intensified fighting close to the Syrian border settlement of Azamarin, with mortar and machine-gun fire clearly audible from the Turkish side. Wounded civilians, some of them in makeshift boats full of women and children, could be seen crossing the narrow 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 especially troublesome because Turkey is a member of NATO, which considers an attack on one member an attack on all, implicitly raising 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 Syria neighbors 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. Skirmishes have been reported between Syrian troops and Jordanians guarding their northern border, and Jordan is worried that the porous frontier could become a conduit for Islamic militants joining the anti-Assad struggle.
At the same time, Mr. Assad's government appears to have hardened its position over the already remote possibility of a truce with the rebels. On Wednesday the government rejected a proposal made a day earlier by Ban Ki-moon, the United Nations secretary general, that Mr. Assad take the first step by declaring an immediate unilateral cease-fire, to be followed by a matching step from his armed opponents.
Jihad Makdissi, a spokesman for the Syrian Foreign Ministry, said in response that the insurgents must stop shooting first. In a statement reported by the official Syrian Arab News Agency, Mr. Makdissi said his government had told Mr. Ban he should send emissaries to the countries arming the insurgents, and urge them "to use their influence to stop the violence from the other side, then informing the Syrian side of the results."
Anne Barnard reported from Beirut, and Sebnem Arsu from Hatay, Turkey. Reporting was contributed by Christine Hauser and Rick Gladstone from New York, Alan Cowell from Paris, and Hwaida Saad from Beirut.
This article originally appeared in The New York Times.