Fighting between Syrian rebel and government forces eased on Saturday as a Russian-led effort to shore up a ceasefire took effect, although battles continued on an important frontline near Hama, a rebel commander and war monitor said.

The deal to create "de-escalation" zones in the major areas of conflict in western Syria took effect at midnight. The initiative was proposed by Russia, President Bashar Al Assad's most powerful ally, with the support of Turkey, which backs the opposition. Iran, Assad's other major ally, also backed it.

Political and armed opposition groups have rejected the proposal, saying Russia has been unwilling or unable to get Assad and his Iranian-backed militia allies to respect past ceasefires. The Syrian government said it backed the proposal but said it would continue to fight what it called terrorist groups across the country.

The Syrian Observatory for Human Rights said there had been a reduction in fighting across Syria since the deal came into force, but warned it was too early to say whether it would last.

"The reduction in violence must be clear and lasting," Observatory Director Rami Abdulrahman said.

The rebel commander said the general level of violence was reduced, but added: "Regime attempts (to advance) in the Hama countryside continue."

The Observatory said it had not recorded any deaths as a result of fighting in the four zones since midnight, but there had been some violations.

Breaches were seen mainly in northern Hama province, where Syrian government and allied forces have taken territory from rebels in recent weeks.

Fighter jets fired at the rebel-held village of Al Zalakiyat and nearby positions in the northern Hama countryside, where the combatants exchanged shelling, the war monitoring group said.

The Observatory said government forces shelled the nearby towns of Kafr Zita and Latamneh.

Mohammed Rasheed, a spokesman for the Jaish Al Nasr rebel group based in Hama, confirmed that fighting had broken out after midnight.

Rasheed said rebel-held Idlib province to the north of Hama was almost completely quiet, but the attacks, which included barrel bombs, were focused on the northern Hama frontline area. "The bombardment has not stopped, it is no different from before," he said.

Iran and Turkey agreed on Thursday to a Russian proposal for de-escalation zones in Syria. The text of the memorandum was published by the Russian foreign ministry on Saturday.

The agreement said four de-escalation zones would be established in Syria for a period of six months which could be extended if the three signatory countries agree. Weaponry and air strikes were not to be used in those zones by combatants, the text said.

The agreement also included creating conditions for humanitarian access, medical assistance and the return of displaced civilians to their homes.

This initiative is the most serious effort to reduce violence and shore up a ceasefire first declared in December.

Four zones to be set up for six months

> Four de-escalation zones will be set up for a period of six months.
> These zones could be extended if the signatory countries agree.
> Largest zone will be set up in northern Syria and includes Idlib province and adjoining districts of Hama, Aleppo and Latakia.
> Other three zones will be set up in Homs province, Ghouta region and along the Jordanian border.