Violence in Syria continued on Tuesday in several cities, claiming the lives of civilians and army personnel, as the head of the UN advance observer team to Syria described the mission as "intractable."

Four Law-enforcement members and one civilian were killed Tuesday when an "armed terrorist group" threw a bomb at a bus transporting the members at al-Shaar roundabout in northern Aleppo province, state-run SANA news agency reported.

The civilian was a private car driver who was in the place at the time of attack, SANA said, adding that nine other law- enforcement members and seven civilians were injured in the attack.

In southern Daraa province, a civilian was killed by gunmen, who stormed the house of an army member and opened fire indiscriminately, said SANA. The report added that another " terrorist" group kidnapped the mayor of al-Mseifra town in Daraa and took him to an unknown destination.

In northern Idlib province, SANA said a judge was abducted by gunmen at al-Qournini town, giving no further details.

On the opposition side, activists said that violence continued in the country despite the cease-fire. They claimed that government troops fired mortars and shells at the neighborhoods of Khaldiyeh and Bayada in central Homs province on Tuesday morning.

According to the Britain-based Syrian Observatory for Human Rights, at least two people were killed Tuesday when army tanks pounded the southern town of Busra al-Harir, known as a stronghold of the rebel Free Syrian Army.

Meanwhile on Tuesday, the head of the UN advance observer team to Syria said that the mission is not an easy process, stressing that there should be adequate planning and coordination.

Moroccan Col. Ahmed Himmiche, the mission's chief, noted that all concerned parties should coordinate their efforts towards the success of the mission. He later in the day traveled to Daraa and met with the local governor.

"We already began our task ... we are moving ahead now with liaison with the Syrian authorities and we hope in the future to liaise with the other part," Himmiche said after returning to Damascus. "Our team is increasing and that is good for us."

The five-member advance team of UN observers arrived in Damascus overnight Sunday to monitor the implementation of the cease-fire which was brokered by UN-Arab League joint special envoy Kofi Annan and went into effect last Thursday.

Their arrival came a day after the UN Security Council unanimously approved the observer mission. The advance team will be followed by other batches of observers and the total number of monitors may eventually reach 250.

Syrian private media said on Monday that the UN team had met with Syrian officials in order to reach an agreement on a protocol that should regulate the work of observers on the ground.

Annan's spokesman Ahmad Fawzi said on Monday that the mission had started setting up headquarters and reaching out to the Syrian government and the opposition to ensure that both sides understand the role of the mission.

Meanwhile, Russia on Tuesday urged all Syrian parties to maintain ceasefire and abide by Annan's peace plan.

"The Russian side pointed out the importance of preserving ceasefire and strict and consistent fulfillment by the Syrian government and all other parties of the obligations stemming from Annan's plan," the Russian Foreign Ministry said in a statement after talks between Deputy Foreign Minister Mikhail Bogdanov and Syrian Ambassador to Russia Riyad Khaddad in Moscow, where Russian Foreign Minister Sergei Lavrov and the visiting Syrian opposition delegation reached consensus on approaches to the Syrian crisis during their talks.

However, in the meantime, the head of the rebels Free Syrian Army, Riadh Asaad, reportedly charged that the Syrian government would not allow the observers to move freely, and called on the international community to press Damascus to fully implement the UN truce.