Syrian activists called on Sunday for the Assad government to engage in serious talks on political transition and for the United Nations to strengthen the fragile ceasefire as violence engulfed parts of the country.

U.N. mediator Staffan de Mistura said a militant attack in Homs on Saturday was a deliberate attempt to wreck the Geneva peace talks, while the warring sides traded blame and appeared no closer to actual negotiations.

"Our hopes are not high given the incidents on the ground and the continuous violations by the regime forces and its backers of the ceasefire," Mutasem Alysoufi of 'The Day after Syria' campaign that supports democratic transition, told Reuters in Geneva.

Warplanes bombed rebel-held areas around several Syrian cities on Sunday including in the al-Waer district of Homs, and in towns around Damascus, the Syrian Observatory for Human Rights, a Britain-based war monitor, said.

One person was killed in the Damascus suburb of Douma and three in al-Waer, the Observatory said, while shells and rockets were launched at insurgent districts in Deraa and Idlib provinces. Rebels fired several shells at a suburb of government-held Aleppo.

Under Security Council resolution 2254, de Mistura is meant to develop a plan to monitor the ceasefire and sanction those who violate it, Alysoufi said. "So this is a duty of the U.N."

Alysoufi added that he did not believe the government delegation, led by Syria's U.N. Ambassador Bashar Ja'afari, wanted to engage in serious political talks.

"They are gaining more time and continuing their military strategy on the ground," he said.

"NOTHING AGREED UNTIL EVERYTHING AGREED"

De Mistura handed a working paper on procedural issues to delegations on Friday but there appears little prospect of moving to the key political issues - a new constitution, U.N.-supervised elections and accountable governance.

He met on Sunday with two opposition groups that curry favour with Russia, President Bashar al-Assad's main backer.

The U.N. envoy has indicated to the High Negotiations Committee (HNC), which is leading the main opposition delegation, that he would like to unify the disparate groups to facilitate face-to-face talks with the government to end the nearly six-year-old conflict.

Jihadi Makdissi, who heads opponents from the "Cairo" platform, gave no sign that a unified delegation could emerge, but said he was coordinating with the HNC.

"We are not a fragmented opposition, we are merely diverse," Makdissi, a former Syrian foreign ministry spokesman, told reporters.

According to de Mistura's paper, seen by Reuters, the agenda is based on resolution 2254 with focus on the three political issues and would be discussed in working groups.

Nothing would be agreed until everything is agreed, the paper says. The aim of this round was to forge a "deeper shared understanding" of how to proceed in future rounds, it said.

It also reiterates that issues related to the ceasefire and fighting terrorism should be the focus of separate talks in the Kazakh capital of Astana, which are sponsored by Russia and Turkey with the support of another Assad backer, Iran.

Dr. Mazen Kewara, director of the Turkey operation of the Syrian American Medical Society (SAMS), said it was important to have a U.N. ceasefire monitoring mechanism and safe, unhindered access for aid workers to reach Syrian civilians.

"We prefer a ceasefire, a complete ceasefire in all Syria.

"But if it's not possible we would like to see safe areas where we can provide our services freely with complete protection for civilians and our health care workers," he said.