Syrian forces opened fire on refugees who fled into a Turkish camp as violence spilled over the country's borders ahead of a fragile ceasefire deadline.
Two Syrian refugees and a Lebanese reporter died in cross-border skirmishes when troops fired into Turkey and Lebanon in separate incidents.
Reuters speculated the shootings in Turkey may have been the result of stray bullets fired during a fight with rebels on the border village of Salama, inside Syria, in which six security forces were killed and eight rebels were injured.
Some news agencies said up to 15 people were injured, while officials quoted by the New York Times claimed that the three wounded people - two Syrians and a Turkish translator - were shot while trying to rescue two civilians who had already been killed.
The shootings took place early on Monday, according to Turkish officials, in the camp near the town of Kilis in the southwest of the country.
Turkey's deputy foreign minister said the ceasefire deadline would become "void" saying: "After Kofi Annan's visit tomorrow a new stage will start."
In a statement, UN Secretary General Ban Ki-moon said he was "alarmed" by the reports and had spoken to the Turkish foreign minister.
"The Secretary-General strongly deplores today's fatal cross-border shootings from Syria into Turkey, as well as into Lebanon."
Turkey has now asked Syrian officials to explain themselves, and summoned a Syrian diplomat in Ankara to give more information.
More than 24,000 Syrians have crossed the border into Turkey since the year-long uprising descended into widespread violence, activists have said.
Up to 50 people were reported dead by activists across the country on Monday.
Meanwhile on the Lebanese border security officials said that a journalist was shot and killed by gunfire.
The dead man was a cameraman working for Al Jadeed, a Lebanese TV station. He was reportedly filming in the Wadi Khaled area on Monday when the shots were fired from the Syrian side of the border, according to news agency reports.
According to Lebanese journalists the Syrian government security forces were behind the attack.
One survivor told local TV that they asked the army not to shoot but they fired anyway.
UN and Arab League special envoy Kofi Annan will reportedly visit the camps in Turkey for a "few hours" ahead of a trip to Iran on Tuesday, a diplomatic source told Al Jazeera.
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The shootings came as a plan to end the violence in Syria appeared to be in ruins, after rebels rejected last-minute demands by President Bashar al-Assad ahead of a critical deadline.
The ceasefire plan, brokered by UN and Arab League special envoy Kofi Annan, had been endorsed by the Security Council, including Russian and China, and agreed in principle by the government and the rebels.
It was intended the plan would begin with a withdrawal of government troops from civilian areas on Tuesday and be followed by a ceasefire 48 hours later.
But the Syrian Foreign Ministry changed its demands on Sunday and said it would need written guarantees from the rebels that they would put down their weapons.
"To say that Syria will pull back its forces from towns on 10 April is inaccurate," the ministry said in a statement. "Kofi Annan having not yet presented guarantees on the acceptance by armed groups to a halt of all violence."
That call for fresh guarantees was swiftly rejected by several rebel groups, including the Free Syrian Army , whose commander Riad al-Assad said from Turkey that it did not recognise the regime or trust it would honour any ceasefire.
"We are committed to the Annan plan," Col Riad al-Asaad said. "We will present our guarantees and our commitments to the international community, but not to this regime."
On Monday China called for Syria to honour the agreement, but hopes were low that the violence would draw to a close on deadline.
According to the UN more than 9,000 people have been killed in Syria since the uprising against Assad's government began in March 2011.