The Arab League says it is suspending its controversial monitoring mission in Syria because of an upsurge in violence, although observers will remain in country for now.
Syria has said it is "surprised" and "regrets" the decision.
The mission was set up in December to monitor compliance by Damascus with the league's plan to end bloodshed. But several countries withdrew monitors.
The mission has been criticised as toothless by Syria's opposition.
"Given the critical deterioration of the situation in Syria and the continued use of violence... it has been decided to immediately stop the work of the Arab League's mission to Syria," Secretary General Nabil el-Arabi said in a statement.
He said the issue would now be discussed at the league's council.
"Syria is surprised and regrets the decision taken by el-Arabi to suspend the observer mission after having decided [last week] to extend it for a month," state news agency Sana said on its website.
On Tuesday the league extended the mission for another month. But Saudi Arabia and other Gulf Arab states withdrew their monitors, reducing the total number to 110.
BBC Middle East analyst Sebastian Usher says it is not a big surprise that the mission has been halted, as activists and human rights groups have accused Syrian President Bashar al-Assad of using it to buy time.
Conservative estimates are that about 200 people have been killed since the mission was renewed for a second month on Tuesday.Iran's 'participation'
The diplomatic focus now seems to be switching to the UN Security Council, with speculation that it may vote on a draft resolution on Syria in the coming days - although Russia still opposes the move.
The council met earlier this week to discuss the document drafted by Arab states, the UK, France and Germany.
Those countries supported the league's call for President Assad to hand power to a deputy, who would form a national unity government with the opposition within two months.
The draft resolution also calls for further measures if the Syrian government does not comply.
But Russia, an ally of Mr Assad, has said it will not back the text.
Mr Arabi is due to address the Security Council on Tuesday. He has also been talking directly to Russian officials to try to persuade them to drop their opposition.
Meanwhile, the opposition Syrian National Council (SNC) urged Syria's diaspora across the world to stage protests outside Russia's diplomatic missions, the AFP news agency reports.
The SNC also accused Iran of "participation" in the violent crackdown on protests across Syria.
Activists in Syria say 30 people have been killed across Syria so far on Saturday.
The Local Co-ordination Committees, a network of anti-government groups, reported fighting and shelling in several cities, including Damascus.
The group also said that a colonel and 300 troops had defected to the rebel Free Syrian Army near Damascus.
The UN says more than 5,000 people have been killed since protests against the government of President Assad erupted last March.
Syrian officials say about 2,000 members of the security forces have been killed in the unrest, which has become increasingly violent as defectors from the army join the opposition.
These claims have not been independently verified.
The freezing of the Arab League's observer mission will throw the spotlight even more on the intensifying diplomacy at the UN Security Council.
It signals that regional efforts to halt the carnage have failed and the only alternative is to internationalise the crisis - something Damascus has been eager to avoid. It made clear its strong displeasure at the suspension of the observers.
New York will now be the focus of concerted lobbying, with the western powers, the Arab League and the Syrian opposition all trying to persuade the Russians and others not to block a resolution adopting the Arab peace plan as the way forward.
The bottom line remains that even if a resolution is passed next week, it will not have teeth in the form of mandatory sanctions, far less providing cover for a Libya-style external intervention.
But the pressure is building up, both on Syria itself and on the international allies such as Russia and China who have so far staunchly protected it even from censure at the Council.