The plan's author, international envoy Kofi Annan, visits Turkey and Iran on Tuesday, while Russia hosts the Syrian foreign minister.
"April 10 has become void," Naci Koru, Turkey's deputy foreign minister, said as his country deplored the cross-border shooting that wounded five people in a refugee camp inside Turkey, in the border area which Annan is expected to visit on Tuesday.
Another neighbour, Lebanon, condemned the killing of a local journalist by Syrian soldiers firing over the border.
In Washington, Victoria Nuland, state department spokeswoman, said: "These incidents are just another indication that the Assad regime does not seem at all willing to meet the commitments that it made to Kofi Annan."
Al Jazeera's Anita McNaught, reporting from Antakya in southern Turkey, said the incident signifies "a remarkable escalation in tensions on this already tense cross-border area".
The incident occurred as reports indicated that Syrian forces were trying to prevent refugees from entering Turkey.
Thousands of Syrians are sheltering in eight refugee camps set up in Turkey's three provinces bordering Syria, while others have crossed into Lebanon, Jordan and Iraq.
In Damascus, President Bashar al-Assad said nothing on Monday about whether he would honour his undertaking to Annan to start withdrawing his forces from urban areas on April 10, a deadline that gives him until midnight Syrian time, or 2100 GMT, on Tuesday to comply.
Assad's demand on Sunday for written guarantees of good faith from the rebels - which their leaders rejected out of hand - as well as the hostile actions of Syrian troops on the ground, fueled doubts that Annan's schedule for the full ceasefire to start by 6am local time (03:00 GMT) on Thursday would be respected.
Walid al-Moualem, Syrian foreign ministerm was in Moscow for talks on Tuesday with his Russian counterpart, Sergei Lavrov.
One of Lavrov's deputies said Russia was working with the Syrian authorities to promote talks with the opposition. But he reiterated Moscow's vocal opposition to foreign intervention.
'Battle lines are drawn'
There was also no sign the rebels were holding fire. Opposition activists said their lightly armed and scattered forces had mounted fatal attacks on troops.
One general who defected to the rebels demanded foreign air attacks on Syrian army artillery and tank units.
"You will not need a long air campaign. Seventy per cent of the Syrian military is already out of action and air strikes would be a message to the Syrian people that the international community is really with them," Mustafa al-Sheikh told Reuters news agency at a camp where Turkey is giving refuge to Syrian army officers.
Syrian woman in defiant protest in Damascus
Saying Assad could not rely on conscript troops from the Sunni Muslim majority to follow orders from an officer corps dominated by members of the president's minority Alawite sect, Sheikh urged the US, Turkey and Arab states to mount the kind of strikes used against Yugoslavia in the 1990s.
The aim would be to create safe areas for Syrian army deserters to gather.
Opposition activists, meanwhile, accused government forces of bombarding villages around Idlib in the northwest, as well as attacks elsewhere in the country.
Fresh fighting killed another 48 people on Monday, including 12 soldiers, according to the Syrian Observatory for Human Rights.
The UK-based opposition activists' group said at least 35 civilians were killed in the shelling of the village of Latamna, in the central Hama province.
"The regime had thought that it would control all areas (of rebels by April 10). As this is not happening, it is procrastinating to gain time," Rami Abdel Rahman, a member of the group, said.
"If the Annan plan does not work, no other plan would, and Syria would plunge into a civil war," he told AFP news agency.
