TURKEY'S military chief of staff said his troops would respond with greater force if bombardments from Syria keep hitting its territory.
Several mortar bombs landed outside the Syrian border town of Azmarin and heavy machinegun fire could be heard from the Turkish side as clashes between the Syrian army and rebels intensified along the border.
Plumes of smoke rose into the sky yesterday and cries of "God is greatest" rang out between the bursts of gunfire.
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The Turkish armed forces have bolstered their presence along the 560-mile border and have been responding over the past week to gunfire and shelling coming across from northern Syria, where President Bashar al Assad's forces have been battling rebels who control swathes of territory.
"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 to defend Turkey. He gave no further details, but a senior US defence official said Nato would likely react if Turkey made a request for assistance.
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 Turkish town of Akcakale before flying by helicopter to a military base further east along the frontier.
Turkish Prime Minister Tayyip Erdogan, once an ally of Mr 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," Mr 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 the country yesterday, including six rebels in the strategic town of Maarat al Numan, on the north-south highway linking Aleppo to the capital Damascus.
Activists and rebels said on Tuesday the insurgents seized control of the town after a 48-hour battle but clashes continued in and around Maarat al Numan yesterday.
The Syrian Government said yesterday an appeal by UN Secretary-General Ban Ki-moon for a ceasefire was only acceptable if the rebel forces agreed to abide by it too.
Foreign Ministry spokesman Jihad Makdissi said two previous attempts at a ceasefire had broken down when rebels carried out attacks. UN observers at the time said government forces had also violated the truce.
Mr Ban had asked for a unilateral truce, Mr Makdissi said. Damascus replied that the goal of any truce was to prepare the ground for dialogue, not to seek military advantage.
"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 they can show their commitment to stopping these acts," he said.