Syrian forces were accused of summarily executing 15 civilians, as members of a UN team of observers were evacuated from a shelled town the day after a bomb blast hit their convoy.
Syria's President Bashar al-Assad, however, accused the West of ignoring violence by "terrorists" and said he would demand an explanation from Annan when he visits Damascus later this month.
"After regime forces raided the neighbourhood of Shammas (in the central city of Homs), 15 civilians were found summarily executed," Rami Abdel Rahman of the Syrian Observatory for Human Rights told AFP, in what he branded a massacre.
The overnight killings came a day the Observatory accused troops of another massacre in the town of Khan Sheikhun, in the northwestern province of Idlib, when they opened fire on a funeral procession and reportedly killed 20 people.
At least 12 more people were killed in incidents around the country on Wednesday, the Observatory said, while another four died of wounds suffered during the funeral in Khan Sheikhun.
During Tuesday's funeral, a convoy of UN observers was struck by a homemade bomb, damaging three vehicles but causing no casualties.
Because of blast damage to their car, six members of the team were forced to spend the night with activists in Khan Sheikhun, which came under heavy regime shelling, an activist said.
Annan's office said the UN mission had picked up the six military observers and that they were back at their team site in the central city of Hama.
It was the second roadside bombing involving the military observers' vehicles in less than a week, after six Syrian soldiers escorting a convoy were wounded in a May 9 bombing in Daraa.
The United Nations, which accuses both sides of violating an April 12 ceasefire, reaffirmed its condemnation of any violence against the monitors.
Meanwhile Syrian representatives stayed away from a United Nations anti-torture committee meeting in Geneva that raised allegations of systematic and brutal abuses in the violence-wracked nation.
Committee Chairman Claudio Grossman had written to Syrian authorities to question them about reports of torture in the country where a bloody crackdown on protesters was unleashed in March 2011.
But the call for a special report on the situation went unmet and Damascus questioned the committee's authority to request the document.
Israeli Defense Minister Ehud Barak urged the international community to increase pressure on the embattled regime of Assad, whom he said was "doomed".
Assad's departure would be a "major blow" to Iran as well as Hezbollah and Islamic Jihad, he told CNN.
However Assad, in an interview with Russian state news channel Rossia-24, accused the West of ignoring the real situation in his country.
He complained that, since the arrival of the UN observers there had been an increase in "terrorist attacks" despite a reduction in "direct confrontation" between government forces and their foes.
"The West only talks about violence, violence on the government side. There is not a word about the terrorists. We are still waiting," he said. "I will ask him (Annan) what this is about" when he next visits Syria.
Assad denounced the armed opposition as a gang of "criminals" who he said contained religious extremists, including members of Al-Qaeda. He also said many "foreign mercenaries" from Arab states fighting for the rebels had been killed.
Turning to legislative elections held on May 7, Assad said they showed that the Syrian people "are until now supporting the policy of reform" and "support the institutions of the state."
Voter turnout was only 51.26 percent and, so far, only limited results have been released.
The state news agency SANA reported that 250 people detained in relation to the revolt in Syria but "whose hands were not bloodied" were released on Wednesday.
According to the Syrian Observatory, around 25,000 people are currently detained in Syria. Annan's peace plan calls for the release of those detained in relation to the uprising.
Meanwhile, Annan himself urged Syria to stop delaying an agreement on allowing UN access to more than one million Syrians in need of assistance, saying the process had been "very slow."
More than 12,000 people, the majority of them civilians, have died since the uprising against Assad erupted in March 2011, according to the Observatory, including more than 900 killed since the truce came into effect.
The Washington Post reported on Wednesday that Syria's rebels have seen an influx of arms including anti-tank weaponry, in an effort coordinated with the help of the United States.
Officials in President Barack Obama's administration insist it is not directly supplying the weapons or providing funding, with Gulf states paying for the new arms, the newspaper said.
But Washington has stepped up links with the rebels and regional militaries allying with them, playing a role in the rebel's foreign support network, the report said.© ANP/AFP