NATO said on Tuesday it had drawn up plans to defend Turkey if necessary against any further spillover of violence from Syria's border areas where rebels and government forces are fighting for control.
Rebel suicide bombers struck at President Bashar Assad's heartland, attacking an Air Force Intelligence compound on the edge of Damascus, insurgents said. Activists living nearby said the bombing caused at least 100 casualties among security personnel, based on the ambulances that rushed to the scene.
"Assad...is only able to stand up with crutches," Turkish Prime Minister Tayyip Erdogan, once a close ally of Assad, told a meeting of his ruling AK Party.
"He will be finished when the crutches fall away."
Erdogan, reacting to six consecutive days where shells fired from Syrian soil have landed on Turkish territory, has warned Ankara will not shrink from war if forced to act. But Ankara has also made clear it would be reluctant to mount any major operation on Syrian soil, and then only with international support.
Syrian forces and rebels have clashed at several sites close to the Turkish border in the last week. There has been no sign of any major breakthrough by either side, though activists said rebels killed at least 40 soldiers on Saturday in a 12-hour battle to take the village of Khirbet al-Joz.
It was not clear whether the shells landing on the Turkish side were aimed at Turkey or simply the result of government troops overshooting as they attacked rebels to their north.
NATO Secretary-General Anders Fogh Rasmussen said in Brussels the 28-member military alliance hoped a way could be found to stop tensions escalating on the border.
"We have all necessary plans in place to protect and defend Turkey if necessary," he said.
Just outside Hacipasa, a village nestled among olive groves in Turkey's Hatay border province, the sound of mortar fire could be heard every 10-15 minutes from around the Syrian town of Azmarin. A Syrian helicopter flew high over the border.
Villagers used ropes and small metal boats to ferry the injured across a river no more than 10 meters wide into Turkey. On the Syrian side, men wearing surgical masks and gloves tended to the wounded on mats laid on the ground.
"They are burning houses in the town," said Musana Barakat, 46, an Azmarin resident who makes frequent trips between the two countries, pointing at plumes of thick smoke in the distance.
"There are rebels hiding in and around the town and they are going to make a push tonight to drive Assad's forces out," he said, a Syrian passport sticking out of his shirt pocket.
A crowd gathered around a saloon car, the blood-stained body of a man who had been pulled wounded from the fighting slumped across its back seat. Those with him said he had been rescued alive but died after being brought over the border.
Turkish President Abdullah Gul said on Monday the "worst-case scenarios" were now playing out in Syria and Turkey would do everything necessary to protect itself.
Gul and Erdogan, in seeking Western and Arab support, have repeatedly warned of the dangers of fighting in Syria spilling over into a sectarian war engulfing the entire region.
Turkey's chief of general staff General Necdet Ozel flew by helicopter to several bases in Hatay province on Tuesday, part of Turkey's 900-km (560-mile) border with Syria.
U.N. special envoy Lakhdar Brahimi will go to Syria soon to try to persuade President Bashar al-Assad's government to call an immediate ceasefire.
The militant Islamist group al-Nusra Front said it had mounted the suicide attack on the air force intelligence building in Damascus because it was used a centre for torture and repression in the crackdown on the revolt against Assad.
"Big shockwaves shattered windows and destroyed shop facades. It felt as if a bomb exploded inside every house in the area," said one resident of the suburb of Harasta, where the compound was located.
But much of the fighting in the 18-month-old uprising has concentrated around the border area.
The shelling of the Turkish town of Akcakale last Wednesday, which killed five civilians, marked a sharp escalation.
Turkey has been responding in kind since then to gunfire or mortar bombs flying over the border and has bolstered its military presence along the frontier.
"We are living in constant fear. The mortar sounds have really picked up since this morning. The children are really frightened," said Hali Nacioglu, 43, a farmer from the village of Yolazikoy near Hacipasa.
A mortar bomb landed in farmland near Hacipasa on Monday.
Unlike the flat terrain around Akcakale, the border area in Hatay is marked by rolling hills with heavy vegetation. Syrian towns and villages, including Azmarin, are clearly visible just a few kilometers away.
"It's only right that Turkey should respond if it gets fired on but we really don't want war to break out. We want this to finish as soon as possible," said Abidin Tunc, 49, a tobacco farmer also from Yolazikoy.
NATO member Turkey was once an ally of Assad's but turned against him after his violent response to the uprising, in which activists say 30,000 people have died.
Turkey has nearly 100,000 Syrian refugees in camps on its territory, has given sanctuary to rebel leaders and has led calls for Assad to quit.
Additional reporting by Khaled Yacoub Oweis in Amman, Adrian Croft in Brussels, John Irish in Paris; Writing by Nick Tattersall; Editing by Daren Butler and Ralph Boulton