By Adrian Blomfield, Middle East Correspondent
The unprecedented cross-border battles, which claimed the lives of two Syrian refugees and a Lebanese reporter, came just hours before a ceasefire deadline came into force.
The Turkish government, which has already threatened to take military action to protect its border, summoned Syriaâ€™s envoy to Ankara to demand an official explanation.
According to Turkish officials, Syrian troops pursued rebels across the border following an attack on a checkpoint that killed six soldiers. Two Syrian nationals, described as civilian, were subsequently shot dead as they tried to escape to the nearby Killis refugee camp.
It was unclear whether the two victims had already crossed the frontier but five more people â€“ including a Turkish policeman â€“ were later shot and wounded, this time indisputably on Turkish soil.
Shortly afterwards, Lebanonâ€™s Al-Jadeed Television network said that one of its cameramen, Ali Shabaan, died after Syrian troops opened fire on his car on the Lebanese side of the border. A colleague in the same vehicle was wounded.
Lebanonâ€™s government, dominated by the Shia Islamist group Hizbollah which is funded by the Assad regime, is unlikely to protest too vigorously, although the incident could prove destabilising in a country whose Sunni and Christian populations deeply resent Syrian influence.
Turkey, however, will prove much less malleable. One of the Assad regimeâ€™s most implacable critics in the region, it has been vocal in its scepticism of a peace plan brokered by Kofi Annan, the UN and Arab League special envoy to Syria, that is meant to reach fruition this week.
Under the terms of the initiative, the Syrian government is meant to withdraw its troops and heavy weapons from cities and towns by today, with the rebels required to end all attacks when a full ceasefire is supposed to come into force 48 hours later.
But ever since Mr Annan announced that Syria had accepted the peace plan last Monday, the country has been convulsed by some of its worst bloodshed since the uprising against Mr Assad began a year ago.
Hundreds of people, mostly civilians but also soldiers and rebels, have been reportedly killed as government offensives launched a countrywide offensive to reclaim opposition-held territory.
More than 50 people were killed yesterday, opposition human rights groups said, including 30 civilians in an army artillery assault on al-Latmana, a town in the central province of Hama. Syrian helicopter gunships also attacked a nearby village.
A dozen soldiers were also killed in rebel attacks elsewhere in the country.
The violence of the past week has prompted a surge in the number of refugees fleeing into Turkey, which houses 25,000 Syrians in camps close to the border.
With a further 200,000 civilians displaced within Syria itself, Turkey fears the number of refugees will grow sharply despite the efforts of Mr Assadâ€™s troops to mine the borders.
In response to Turkish warnings that it will soon need international assistance, Mr Annan is due to tour some of the refugee camps today.
Turkeyâ€™s deputy foreign minister, Naci Koru, said the visit would be used to make the point that the peace plan was effectively dead.
â€œApril 10 has become void,â€ Mr Koru said. â€œAfter Kofi Annanâ€™s visit tomorrow a new stage will start.â€
Turkey, which already provides sanctuary to rebel units, has previously proposed mounting a cross-border military incursion into Syria to create a buffer zone but so far has failed to win American support for the idea.
But what little leeway Mr Assad had with his Western detractors is dissipating fast after his government on Sunday backed away from its commitment to withdraw troops by Tuesdayâ€™s deadline, saying it would only do so if it received a written pledge from the rebels that they would also disarm.
â€œAdding new conditions at this stage is totally unacceptable,â€ Baroness Ashton, the European Union policy chief, said.
In a damning indictment that will reinforce Western outrage with the regime, Human Rights Watch said yesterday that it had documented the summary executions by pro-regime forces of at least 85 civilians, including women and children, and 16 captured rebel fighters since late 2011. The group said it believed hundreds more executions had also taken place.
â€œIn a desperate attempt to crush the uprising, Syrian forces have executed people in cold blood, civilians and opposition fighters alike,â€ said Ole Solvang, a researcher for the group. â€œThey are doing it in broad daylight and in front of witnesses, evidently not concerned about any accountability for their crimes.â€
Amateur video footage released by opposition activists in Homs showed a man being kicked and beaten by suspected loyalist militiamen who set fire to his back before slicing open his throat with a knife. The provenance of the clip is unclear and its authenticity could not be established.