Gulf Cooperation Council members say they are withdrawing their observers from Syria, following Saudi Arabia's decision to pull out of the Arab League monitoring mission.
The six-member GCC announced the move in a statement Tuesday. Saudi Arabia said it quit the mission because Syria has not implemented an Arab plan to end the bloodshed of a crackdown against a 10-month uprising against President Bashar al-Assad.
In addition to Saudi Arabia, the GCC includes the United Arab Emirates, Bahrain, Oman, Qatar and Kuwait.
On Monday, the head of the Arab League mission, Sudanese General Mohamed Ahmed Mustafa al-Dabi, said violence in Syria declined after the monitors began work on December 26.
Syria on Monday rejected a new Arab League initiative for President Assad to transfer powers and step down, similar to the transition under way in Yemen. Syrian state media denounced the plan as a "flagrant" violation of Syrian sovereignty.
Qatar said the Arab League would ask the United Nations Security Council to support the initiative. Germany's U.N. ambassador welcomed the plan as a potential "game changer."
Also Monday, European Union foreign ministers approved additional sanctions on Syria, imposing travel bans and asset freezes on another 22 people and eight companies linked to the Assad government.
In violence Monday, Syrian rights activists say army defectors killed five pro-Assad troops in a battle in the central province of Homs. They say government security forces killed at least 10 civilians during operations against centers of protest around the country. Activists also say at least 60,000 people gathered in Douma, near Damascus, for the funerals of 11 people killed by pro-Assad forces in recent days.
The United Nations says violence linked to the uprising has killed more than 5,400 people. Syrian authorities say terrorists have killed about 2,000 security force members since the unrest began.
In Washington, the U.S. State Department expressed concern at reports that Russia may sell military jets to Syria. Spokeswoman Victoria Nuland said any country that is still trading in weapons with Syria "needs to think twice, because they are on the wrong side of history."
The Yak-130 training aircraft could be used for attacks on ground targets and to train pilots on Syria's more advanced fleet of MiG-29 fighters, which it ordered from Russia in 2007. Moscow is one of Assad's few remaining allies, and Syria is its top arms customer.
Some information for this report was provided by AP, AFP and Reuters.