Turkish President Abdullah Gul says the "worst-case scenarios" were now playing out in Syria and Turkey would do everything necessary to protect itself, as its army fired back for a sixth day after a shell from Syria flew over the border.
Gul said the violence in Turkey's southern neighbour, where a revolt against President Bashar al-Assad has evolved into a civil war that threatens to draw in regional powers, could not go on indefinitely and Assad's fall was inevitable.
"The worst-case scenarios are taking place right now in Syria ... Our government is in constant consultation with the Turkish military. Whatever is needed is being done immediately as you see, and it will continue to be done," Gul said.
"There will be a change, a transition sooner or later ... It is a must for the international community to take effective action before Syria turns into a bigger wreck and further blood is shed, that is our main wish," he told reporters in Ankara.
Turkey's armed forces have bolstered their presence along the 900 km (560 mile) border with Syria in recent days and have been responding in kind to gunfire and shelling spilling across from the south, where Assad's forces have been battling rebels who control swathes of territory.
Turkey's Chief of Staff, General Necdet Ozel, travelled to the southern city of Adana and was due to inspect the region patrolled by Turkey's 2nd Army, which protects the border with Syria, the military said on its website.
UN Secretary-General Ban Ki-moon said the escalation of the conflict along the Turkey-Syria border, as well as the impact of the crisis on Lebanon, were "extremely dangerous".
"The situation in Syria has dramatically worsened. It is posing serious risks to the stability of Syria's neighbours and the entire region," he told a conference in Strasbourg, France.
Ban said UN and Arab League envoy Lakhdar Brahimi would be heading back to the region this week.
The exchanges with Turkey mark the most serious cross-border violence in Syria's revolt against Assad, which began in March last year with peaceful protests for reform and has evolved into a civil war with sectarian overtones.
"From now on, every attack on us will be responded to immediately. Every attack that targets our sovereignty, our security of life and property will find its response," Turkish government spokesman Bulent Arinc said after a cabinet meeting.
Parliament last week authorised the deployment of Turkish troops beyond its borders although government officials said the move was meant as a deterrent rather than a "war mandate".
"Turkey will decide itself when the situation necessitates acts mentioned in the motion the parliament passed last week. Nobody should think war will follow a parliament approval ... but we are more sensitive about our independence and sovereignty than most countries," Arinc said.
Fighting further inside Syria also intensified on Monday.
Syrian government forces advanced for the first time in months into the rebel-held Khalidiya district in the besieged central city of Homs, one of 12 districts they have been bombarding for days.
"They have occupied buildings that we were stationed in and we had to evacuate," a rebel fighter told Reuters by Skype.
Skirmishes on the Syrian side of the border have been escalating and it is unclear who fired the shells that have crossed into Turkey.
Damascus has said it fired into Turkey accidentally, but has failed to live up to pledges made last week, after a Syrian shell killed five civilians in the Turkish town of Akcakale, to ensure no more ordnance flies across the border.
Turkey launched its latest retaliatory strike on Monday after a mortar bomb fired from Syria landed in countryside in the Turkish province of Hatay some 150-200 metres inside the district of Hacipasa, a Turkish official told Reuters.
Further east, Syrian rebel sources in Raqqa province, which borders Akcakale, said they had seen five Turkish army trucks full of soldiers patrolling the Turkish side of the border.
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 now died.
Turkey has nearly 100,000 Syrian refugees in camps on its territory, has allowed rebel leaders sanctuary and has led calls for Assad to quit. Its armed forces are far larger than Syria's.
Turkish Foreign Minister Ahmet Davutoglu said at the weekend that a potential leader in an interim Syrian government could be Vice-President Farouq al-Shara.
Reports in August said Shara, a former foreign minister who was appointed vice president six years ago, had tried to defect to neighbouring Jordan, but Syrian state media subsequently said he had never considered leaving.
"The opposition is inclined to accept these names. Farouq al-Shara has the ability to understand the system of the last 20 to 30 years," Davutoglu told the state broadcaster TRT.
"Farouq al-Shara did not get involved in the recent incidents, the massacre, in a very wise and conscientious attitude. But perhaps there is nobody who knows the system better than al-Shara."