Twenty-eight soldiers were among 39 people killed in violence in Syria on Wednesday, a rights group said, as UN observers said they were "morally obliged" to stay in the country despite the risks.

Russia resisted Western pleas to help remove President Bashar al-Assad from power despite escalating hostilities that have battered the UN peace mission that was supposed to start with a ceasefire by both sides from April 12.

The army suffered heavy losses in two northwestern provinces on the Turkish border, where rebel fighters of the Free Syrian Army have intensified their operations in recent weeks.

At least 20 soldiers were killed in fierce clashes with rebel fighters in Latakia province, the Syrian Observatory for Human Rights said.

Five rebels were also killed in the clashes that began late Tuesday and continued through dawn on Wednesday in a region known as the Kurdish Mountain near the border, the Britain-based watchdog said.

"The majority died in direct fighting with the rebels, while other soldiers were killed in a rebel attack on two buildings, which the army was using to launch mortar attacks against the Kurdish Mountain," Observatory head Rami Abdel Rahman told AFP.

Dozens of soldiers were also wounded, Abdel Rahman said.

In Idlib, province, five soldiers were killed when a car bomb exploded at their checkpoint overnight. Explosions and shooting were also heard in the town of Maaret al-Numan, the Observatory said.

In the central province of Hama, clashes in the town of Kernaz left three soldiers dead. Shelling by government troops also killed a man and his wife.

"Unidentified gunmen assassinated a Shiite cleric in Sayyida Zeinab," the Observatory said, referring to an area of south Damascus that houses a revered Shiite Muslim shrine of the same name and is home to many Iraqi refugees.

Northeast of the capital, troops shot a civilian dead at a checkpoint in Harasta, while a rebel officer died of wounds sustained in an earlier attack in the town.

In the eastern city of Deir Ezzor, sniper fire killed a civilian in the Al-Qasur neighbourhood, the watchdog said.

With the death toll exceeding 14,400 since the uprising against Assad's rule erupted in March 2011, according to Observatory figures, the head of the UN Supervision Mission in Syria told the Security Council that the nearly 300 unarmed UN observers were "morally obliged" to stay.

"We are going nowhere," Major General Robert Mood told reporters after the closed meeting.

Highlighting the dangers faced by them, Mood told the meeting UN vehicles had been hit 10 times by "direct fire" and hundreds of times by "indirect fire." He said nine UN vehicles had been hit in the past eight days alone.

But he insisted that the mission's decision on Saturday to suspend its operations until security conditions improve did not mean an "abandonment" of Syria. UNSMIS was "morally obliged not to turn away," he was quoted as saying.

In a statement issued after the meeting, Mood said the first condition for a resumption of operations was a "significant" reduction in violence.

In addition, there needed to be a commitment by both the government and the opposition to the observers' safety and security, as well as their freedom of movement.

"The government has expressed that very clearly in the last couple of days. I've not seen the same clear statements (from) the opposition yet," he said.

The mission's mandate ends on July 20, and Western governments have warned that it will be hard to agree a renewal if the violence continues to intensify.

US President Barack Obama said he had told Russian President Vladimir Putin and Chinese President Hu Jintao that Assad could no longer remain in power after the massacres of large numbers of civilians.

"I wouldn't suggest that at this point the United States and the rest of the international community are aligned with Russia and China ... but I do think they recognise the grave dangers of all out civil war," Obama said.

But Putin reiterated Russia's opposition to talk of regime change in Syria after their talks at a G20 summit in the Mexican beach resort of Los Cabos.

"We believe that nobody has the right to decide for other nations who should be in power and who should not," the Russian leader told reporters.

"It is not changing the regime that is important, but that after changing the regime, which should be done constitutionally, violence is stopped and peace comes to the country," he said.

The owners of a Russian cargo ship reportedly carrying attack helicopters to Syria, which was turned back from British waters after its insurance coverage was revoked, denied that the vessel was transporting anything illegal.

"Reports about illegal actions of the MC Alaed's owner are inaccurate," Russia's Femco cargo line said. "The vessel is currently making a regular commercial voyage, in full accordance with international norms and rules."