°®Âþµº

Saudi jets on Friday arrived at a Turkish base to join the air campaign against the Islamic State group [IS] in Syria, only hours before a partial "ceasefire" is to come into effect.

Four F-15 jets landed at Incirlik air base in the Adana province in southern Turkey; ground personnel and equipment arrived at the base earlier this week.

"It is possible that Saudi Arabia and Turkey could intervene [in Syria] without warning, especially since the US has hinted at a so-called 'plan B' should the ceasefire fail," °®Âþµº's Gulf correspondent Badr al-Rashid said.

"John Kerry has said this plan would include providing greater military support to rebel groups," Rashed added.

The partial truce to end hostilities in Syria, brokered by Russia and the United States, is due to begin at midnight on Friday. The deal excludes IS and the al-Qaeda-affiliated al-Nusra Front.

Turkey on Friday expressed alarm over the viability of the ceasefire as the Syrian regime and its ally Russia pressed ahead with an offensive.

Also on Friday, Russian jets intensified attacks on rebel positions, hours before a cessation of hostilities was due to come into force.

Russia said it was continuing to bomb "terrorists" in parts of Syria.

The two Sunni Muslim powers, Saudi Arabia and Turkey, both see the ousting of Syrian President Bashar al-Assad as essential to ending Syria's five-year civil war and are bitterly critical of Iran and Russia's support of the Syrian regime.

Ankara and Riyadh have both said they were in favour of a ground operation in Syria, but only if it is conducted in coordination with Saudi Arabia as well as Western and Gulf members of the anti-IS coalition.

Turkish Foreign Minister Mevlut Cavusoglu on Thursday said: "We have since the beginning argued for the necessity of ground operations and all kinds of strategic moves to be carried out in addition to the air campaign."