RIYADH (AFP) - Saudi King Abdullah on Sunday sharply criticised religious extremists, vowing not to let "a handful of terrorists...terrify Muslims" in a speech marking the start of the Muslim holy fasting month of Ramadan.

Islam is "religion of unity, fraternity and mutual support" but some people "lured in by false calls... are confusing reform with terrorism", the monarch said, in comments carried by state news agency SPA.

"Their goal is to sow discord among Muslims," he said in an apparent reference to insurgents from the Islamic State of Iraq and Syria (ISIS). The powerful jihadist group has spearheaded an offensive by Sunni militants in Iraq since June 9, wresting control of northern cities and capturing vast swathes of territory.

ISIS operates in both Syria and Iraq and aims to establish an Islamic state straddling the border of those two countries, but their lightning advance in Iraq also poses a threat to Jordan and Saudi Arabia. The ultra-conservative Sunni Gulf kingdom - home to Mecca and Medina, Islam's holiest sites - shares an 814-kilometre border with Iraq.

"We will not allow a handful of terrorists, using Islam for personal aims, to terrify Muslims or undermine our country and its inhabitants," King Abdullah said. "We are continuing, with God's help, to face and tackle this scourge," he said.

The Saudi monarch also wished Muslims "security, prosperity and stability" over Ramadan, which began in most countries on Sunday.

During Ramadan, which is sacred to Muslims because it is during that month that tradition says the Koran was revealed to the Prophet Mohammed, believers abstain from eating, drinking, smoking and having sex from dawn until sunset.