Turkey scrambled fighters to force a Syrian passenger plane to land on Wednesday, suspecting it of carrying military equipment from Moscow, while Turkey's military chief warned of a more forceful response if shelling continued to spill over the border.
Military jets escorted the Damascus-bound Airbus A-320, carrying around 30 passengers, into the airport in Ankara hours after Turkey's chief of staff said his troops would respond with greater force if bombardments from Syria kept hitting Turkish territory, Turkish state-run television said.
"We are determined to control weapons transfers to a regime that carries out such brutal massacres against civilians. It is unacceptable that such a transfer is made using our airspace," Turkish Foreign Minister Ahmet Davutoglu said.
"Today we received information this plane was carrying cargo of a nature that could not possibly be in compliance with the rules of civil aviation," he said in Athens during an official visit, in comments broadcast live on Turkish television.
Mr. Davutoglu said Turkey was within its rights to investigate planes suspected to be carrying military materials and that the plane would be allowed to continue if it was found to be clean. He declined to comment on what the banned materials might be.
He also said Syrian airspace was no longer safe and that Turkish passenger planes should not fly there. A witness at the border saw at least one passenger plane turn around as it approached Syria and head back into Turkey on Wednesday.
More than 18 months into the battle for Syria, an estimated 30,000 people are dead and the country is disintegrating. Rebels are outgunned by the government but can still strike at will, and President Bashar al-Assad has assumed personal command of his forces, convinced he can prevail militarily.
Meanwhile, the conflict threatens to spill over borders and ignite a wider Middle East war, drawing in neighbouring states and pitting Sunni Muslim states against Syria's rulers and their allies including Shiite Iran.
Russia, from where the Syrian plane took off, is one of Mr. al-Assad's closest remaining alliesand has blocked tougher United Nations resolutions against Damascus. A spokeswoman for Vnukovo airport was quoted by Interfax news agency as saying 25 people were on board and that it was a charter plane.
Turkey's armed forces have bolstered their presence along the 900-kilometre border and have been firing over the past week in response to gunfire and shelling coming across from northern Syria.
Several mortar bombs landed outside the Syrian border town of Azmarin and heavy machine-gun fire could be heard as clashes between the Syrian army and rebels intensified. "We responded but if it continues we will respond with greater force," state television TRT quoted Turkey's Chief of Staff, General Necdet Ozel, as saying.
It is not clear whether the shells that have hit Turkish territory were aimed to strike there or were due to Syrian troops overshooting as they attacked rebel positions.
General Ozel visited the family of five civilians killed last week by a Syrian mortar strike in the town of Akcakale, before flying to a military base further east.
Scores of Syrian civilians, many of them women with screaming children clinging to their necks, crossed a narrow river marking the border with Turkey as they fled the fighting in Azmarin and surrounding villages.
The Swiss government on Wednesday toughened rules to prevent weapons sold to one country from being re-exported to areas of conflict after Swiss-made grenades turned up in Syria.
An investigation found that the United Arab Emirates had given Jordan grenades sold by Switzerland in 2003 and 2004 which later were channelled to Syria.
U.S. Defence Secretary Leon Panetta said on Wednesday U.S. military planners were in Jordan to help the government grapple with Syrian refugees, bolster its military capabilities and prepare for any trouble with Syrian chemical weapons stockpiles.