Major General Robert Mood, head of the UN observer mission in Syria, has called on all sides to "stop the violence" upon his arrival in Damascus.
Mood, a Norwegian with experience from several UN missions, will take charge of an advance team of 16 monitors trying to salvage a peace plan brokered by special envoy Kofi Annan that aims to end the country's 13-month-old crisis.
"To achieve the success of the Kofi Annan plan, I call on all sides to stop violence and help us continue the cessation of armed violence," Mood told reporters on Sunday.
Under the plan, a ceasefire between government forces and opposition fighters is supposed to lead to talks between President Bashar al-Assad and the opposition on a political solution to the conflict that the UN says has killed more than 9,000 people.
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The observer mission is to be expanded to 300 monitors in the coming months.
"Thirty unarmed observers, 300 unarmed observers, even 1,000 unarmed observers cannot solve all the problems," he said. "I call on everyone to help us and cooperate with us in this very challenging task ahead."
Opposition activists say more than 360 people have been killed since the ceasefire officially went into force on April 12.
In the latest reports of violence, armed fighters fired rocket-propelled grenades at the Central Bank building in Damascus and also attacked a police patrol in the capital, wounding four police, state television said on Monday.
It said the attack on the Central Bank by "an armed terrorist group" caused only slight damage, and security forces were chasing the attackers of the police patrol. Activists in the capital reported hearing several explosions and gunfire.
The UK-based Syrian Observatory for Human Rights said that 25 people were killed on Sunday, including four soldiers who died in a blast at a military centre in Aleppo province.
The Observatory said 14 people were killed in the village of Hamadi Omar in the central Hama province. Two civilians were shot dead by snipers in the district of Juret al-Shayah in Homs, the group said.
But an activist in Homs told Reuters news agency that violence had dropped sharply since the observers deployed a permanent two-man team to the city last week.
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"There are still violations, but the shelling and mortar fire has stopped," Karam Abu Rabea said. "We have insisted that the observers stay in Homs because we know if they leave [the attacks] will continue."
Meanwhile, the state-run news agency said observers toured Homs' al-Khaldiyeh district, which has seen heavy government shelling and clashes between Syrian forces and opposition fighters.
The government says 2,600 members of the security forces have died at the hands of anti-Assad fighters, and has accused the UN of turning a blind eye to "terrorist acts".
In the wake of a series of bombings in the capital, Damascus, state newspapers on Sunday charged that al-Qaeda was operating in Syria and carrying out its trademark suicide bombings with the support of Washington and some Arab countries.
"The recent terrorist suicide bombings in several areas of Syria are not the first signs of al-Qaeda's presence in Syria," but now there is "clear evidence" because of the methods and choice of targets, government daily Tishreen said.
"Washington has a history of supporting terrorism" and "this is being repeated in Syria ... where terrorist groups are being funded by some Arab countries and supported by the West," the editorial said.
A group called "al-Nusra Front" claimed responsibility on Sunday for a blast which state media said killed at least nine people in al-Midan neighbourhood on Friday.
The US-based SITE Monitoring Service said the group posted its claim on the Shumukh al-Islam site which is generally used by al-Qaeda for posting its statements.
The group named the bomber as Abu Omar al-Shami and said he detonated his explosives amid 150 members of the Syrian security forces who were gathered outside the Zain al-Abideen mosque.
The statement said Friday's bombing targeted the "aggressors who surround the houses of God" to attack worshippers after weekly prayers.
Al-Nusra Front had previously claimed the February 12 twin bombings in Aleppo and the January 6 bombing in al-Midan, SITE said.