Turkey has confirmed it is deploying more fighter jets to an airbase close to the border with Syria, amid artillery exchanges along its tense southeastern border with Syria.
The announcement came amid reports of fierce fighting in the northern Idlib province on Tuesday where Syrian rebels are trying to take control of a strategic town.
"Assad ... is only able to stand up with crutches," Recep Tayyip Erdogan, the Turkish prime minister, who was once a close ally of Syrian President Bashar al-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 said Turkey will not shrink from war if forced to act.
But Turkey has also made clear it would be reluctant to mount any major operation on Syrian soil, and then only with international support.
At least 25 additional F-16 fighter jets were deployed at Turkey's Diyarbakir airbase late on Monday.
Meanwhile, the NATO secretary-general said that Turkey can rely on the alliance, which has "all necessary plans in place to protect and defend Turkey if necessary".
Anders Fogh Rasmussen cautioned against the dangers of the conflict in Syria escalating, saying alliance member Turkey had shown commendable restraint in response to the shelling of its border area.
"I would like to commend the Turkish government for the restraint it has shown in its response to the completely unacceptable Syrian attacks," he said as he went into a two-day NATO defence ministers meeting.
"Obviously Turkey has a right to defend herself within international law."
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Rasmussen noted NATO has "all necessary plans in place to protect and to defend Turkey if necessary".
Turkey as an alliance member has the right to invoke military help in response to an attack on its territory under Article V of NATO's constitution but has so far invoked only Article IV, which involves consultations.
"We hope it won't be necessary, we hope that both countries will show restraint and avoid an escalation of the crisis," Rasmussen said.
Reports from Ankara on Tuesday said Turkey's top military commander, General Necdet Ozel, had inspected troops in southeastern Hatay province near the Syrian border, a day after a Syrian shell landed in a nearby town.
Syrian shells last week killed five people in a border village in Hatay, sparking a series of retaliatory strikes and a firm message of support from NATO for Turkey.
The Turkish parliament on Thursday gave the government the green light to use military force against Syria if necessary.
Officials said on Monday that the Syrian conflict was not on the defence ministers' agenda but that the issue was likely to be discussed informally.
Against this backdrop of escalating border tensions, Syrian rebels took control on Tuesday of Maarat al-Numan, a strategic town in Idlib on the highway linking Damascus with the country's second city, Aleppo, according to the Syrian Observatory for Human Rights (SOHR).
"Regular forces pulled back from all of their checkpoints around Maarat al-Numan, except for one at the entrance of the town," the UK-based opposition network's Rami Abdel Rahman said.
"This is a strategic location on the route from Damascus to Aleppo. All the regime reinforcements headed to Aleppo must pass through Maarat al-Numan."
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Opposition activists in Syria told Al Jazeera that rebel fighters had captured most of the army's checkpoints in the area, but that fighting was still under way for other government positions on Tuesday night.
Al Jazeera is unable to independently verify reports of violence, as the Syrian government has placed strict restrictions on reporting.
In another Syria-related development, Ban Ki-moon, the UN secretary-general, urged the Assad government to declare an immediate truce to bring an end to the conflict that he said had left 20,000 dead over the last 19 months.
"It is unbearable for the [Syrian] people to continue like this," he said on Tuesday.
"That is why I have conveyed to the Syrian government [a] strong message that they should immediately declare a unilateral ceasefire."
Speaking in Paris alongside French President Francois Hollande, Ban said the reaction he had got from the Syrian government had been to ask what opposition forces would do if the regime called a truce.
"That is exactly what I have discussed and I am in the process of discussing with the member states of the [UN] Security Council and the countries in the region," Ban said.
He urged "the opposition forces to agree to this unilateral ceasefire when and if the Syrian government declares it", and he called on countries supplying arms to either side to stop in order to ease the suffering of the Syrian people.
Source: Al Jazeera and agencies