By Nick Tattersall and Ece Toksabay
ISTANBUL (Reuters) - Turkey scrambled fighters and briefly detained a Syrian passenger plane 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," 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.
The Turkish authorities had seized some of the cargo, Davutoglu told reporters later in televised remarks. He said Turkey was within its rights to investigate planes suspected of carrying military materials but declined to say what was in the seized cargo.
The plane and its passengers later left the Turkish capital.
Turkey would continue to investigate Syrian civilian aircraft using its airspace, Davutoglu said.
He also said Syrian airspace was no longer safe and that Turkish passenger planes should not fly there. A Reuters 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 Syria's borders and ignite a wider Middle Eastern war, drawing in neighboring states and pitting Sunni Muslim states against Syria's rulers and their allies including Shi'ite Iran.
Russia, from where the Syrian plane took off, is one of Assad's closest remaining allies and has blocked tougher U.N. resolutions against Damascus.
"Once a week a Syrian Airlines airplane flies from Moscow bound for Damascus," Interfax reported Vnukovo Airport spokeswoman Yelena Krylova as saying. "The plane took off normally, there were no incidents."
Interfax cited her as saying 25 people were on board the chartered plane and it left 20 minutes after its scheduled afternoon departure time.
Turkey's armed forces have bolstered their presence along the 900-km (560-mile) border and have been firing back over the past week in response to gunfire and shelling coming across from northern Syria, where Assad's forces have been battling rebels who control swathes of territory.
Several mortar bombs landed outside the Syrian border town of Azmarin and heavy machinegun fire could be heard as clashes between the Syrian army and rebels intensified.
Plumes of smoke rose into the sky and cries of "God is Greatest" rang out between the bursts of gunfire.
"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.
NATO Secretary-General Anders Fogh Rasmussen said on Tuesday the military alliance had plans in place to defend Turkey.
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. Turkey has provided sanctuary for rebel officers and fighters.
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.
Turkish Prime Minister Tayyip Erdogan, once an ally of Assad but now one of his harshest critics, said in Istanbul that Turkey's objective was to secure peace and stability in the region, not to interfere in Syria's domestic politics.
"We warned Assad. We reminded him of the reforms he should introduce ... unfortunately the Assad regime didn't keep its promises to the world and its own people," Erdogan said.
"Nobody should or can expect us to remain silent in the face of the violent oppression of people's rightful demands."
The Syrian Observatory for Human Rights said 70 people had been killed across Syria on Wednesday, including six rebels in the strategic town of Maarat al-Nuaman, on the north-south highway linking Aleppo to the capital Damascus.
Activists and rebels had said on Tuesday that the insurgents seized control of the town after a 48-hour battle but clashes continued in and around Maarat al-Nuaman on Wednesday.
SYRIANS FLEE ACROSS RIVER
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.
Residents from the Turkish village of Hacipasa helped pull them across in small metal boats.
"The firing started getting intense last night. Some people have been killed, some are lying wounded on the road," said a 55-year-old woman, Mune, who fled Azmarin and sat with several adults and about 20 children outside a house in Hacipasa.
"People want to escape but they can't. Many have settled in a field outside the town and are trying to come," she said, describing how she had helped ferry the children over another point in the river in a metal bowl used for wheat.
Doctors and volunteers set up makeshift first-aid points on both sides of the frontier. A Turkish ambulance and several minibuses and cars waited to take the more seriously wounded to the main city of Antakya or district hospitals.
"Don't take me across, take me back. I want to return and fight," said one man being carried on a stretcher, his T-shirt stained with blood.
A sharp rise in casualties in Syria in the past month indicates the growing intensity of the conflict, which spiraled from peaceful protests against Assad's rule in March 2011 into a full-scale civil war.
The Syrian government said on Wednesday that an appeal by U.N. Secretary General Ban Ki-moon for a ceasefire was only acceptable if the rebel forces agreed to abide by it too.
"We requested the Secretary General to send delegates to the relevant countries, specifically Saudi Arabia, Qatar and Turkey, because those are the countries that finance, shelter, train and arm these armed groups, so that they can show their commitment to stopping these acts," a Foreign Ministry spokesman said.
The Swiss government on Wednesday toughened up rules to prevent weapons sold to one country being re-exported to areas of conflict after Swiss-made grenades turned up in Syria.
The move comes after an investigation found that the United Arab Emirates had given Jordan grenades sold by Switzerland in 2003 and 2004 which later were channeled to Syria.
U.S. Defense 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.
(Additional reporting by Seda Sezer in Istanbul, Gulsen Solaker in Ankara, Jonathon Burch in Hatay and Thomas Grove in Moscow; Writing by Andrew Roche; Editing by Michael Roddy)