Deadly clashes broke out between deserters and Syrian government forces on Saturday as a veteran Norwegian peacekeeper headed to Damascus to take charge of a UN mission overseeing a troubled truce.
Regime troops killed at least 10 rebel fighters in the Damascus region, the Syrian Observatory for Human Rights said.
Separately, the official SANA news agency reported three soldiers and two "terrorists" killed in Aleppo, northern Syria, in clashes between troops and "armed terrorist groups."
An activist said the fighting began as "officers and soldiers of a military base near the presidential palace... deserted with their weapons."
And in what was believed to be the first case of Westerners going missing in the violence-swept country, Budapest said two Hungarians had been kidnapped.
A putative truce, which technically came into effect on April 12, has taken a daily battering, and the European Union on Friday expressed extreme concern about the persistent bloodshed.
The Observatory said four civilians were also killed by security forces on Saturday, two in Damascus province and two in Idlib province in the northwest.
The latest security-related incidents came as Major General Robert Mood was en route for Syria to take the helm of a fledgling monitoring mission after being appointed by UN chief Ban Ki-moon, diplomats said.
Mood takes over a mission already facing major obstacles before the full 300-member force approved by the UN Security Council has even gathered.
He has himself highlighted the "abyss of suspicion" between President Bashar al-Assad and the opposition, in the face of an uprising that has killed more than 9,000 people since March 2011, according to UN figures.
Mood "is on his way to Damascus and arrives there tomorrow," Sunday, international peace envoy Kofi Annan's spokesman Ahmad Fawzi told AFP in Geneva.
Saturday's violence came the day after a suicide car bombing in Damascus that the Observatory said killed two and Syria's state media said 11.
Ten civilians were among 19 people killed in renewed bloodshed on Friday, more than two weeks into a promised ceasefire, the watchdog said.
After the Damascus blast, SANA quoted the interior ministry as saying "it will not tolerate the armed terrorist groups and vowed to strike with an iron fist those who are terrorising citizens."
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The opposition blamed government forces for the bombing and demanded an international inquiry.
"The Syrian National Council condemns this criminal act which is aimed at further undermining the security and stability of our country and at terrorising our people," a statement said.
Meanwhile, the official Syrian press accused UN chief Ban of encouraging "terrorists," the regime term for anti-regime fighters, and Damascus ally Russia said "terrorist groups" needed a "decisive rebuff."
Moscow's statement, which insisted that world powers prevent support from reaching the rebels, came after Lebanon said it had intercepted three containers of weapons destined for rebel forces on a ship originating in Libya.
A Lebanese official said the containers held heavy machineguns, artillery shells, rockets, rocket launchers and other explosives.
Syrian authorities have repeatedly charged that weapons are being smuggled from Lebanon to rebels.
On Saturday, government newspaper Tishrin wrote that Ban "avoids talking about abuses by armed groups and focuses his blame solely on Syria, as usual. He encourages these groups to continue to commit more crimes and terrorist acts."
The Russian foreign ministry said "we are convinced that the terrorists operating in Syria need a decisive rebuff, and that all domestic and outside players need to prevent any support" from reaching the rebel forces.
Meanwhile, a spokesman for Hungarian Prime Minister Viktor Orban told the MTI news agency that two of its citizens were abducted in Syria on Saturday by unknown gunmen, and that Budapest may send agents to the region.
The government in Budapest declined to reveal further details, citing security concerns.
Ahead of Mood's arrival, Norwegian Defence Minister Espen Barth Eide highlighted the "risky" task facing the unarmed observers.
"We therefore have to have realistic expectations in terms of what the observer force can achieve. Still, our hope is that the presence of the observers may help reduce the level of violence in the country," Eide said.
Mood, 54, who has not spoken publicly since being nominated by Ban, highlighted the "abyss of suspicion and violence between the Syrian regime and the opposition" in a recent interview.
"It's worth making the effort," he said of the mission and Annan's peace plan. "The Syrian people deserve to have an opportunity."