Gulf Arab states have announced that they are withdrawing from the Arab League's observer mission in Syria.
The Gulf Co-operation Council also called on the UN Security Council to put new pressure on Damascus to end a violent crackdown against protests.
It came after Syria rejected an Arab League plan for President Bashar al-Assad to step down and hold elections.
Syria's foreign minister said some Arab states had joined a foreign conspiracy to destabilise the country.
Speaking after the GCC's announcement, Foreign Minister Walid Muallem said "a new phase of their plan against Syria" included the West and Arab states.
He said some regional powers were "internationalising" the situation in Syria by asking the UN Security Council to take action against Damascus.
Syria has long held that a foreign conspiracy backing armed groups is behind the country's unrest.
The Local Co-ordination Committees, a network of anti-government activists, said 40 people had been killed across Syria on Tuesday - including 32 in Homs, a major focus of unrest.
The UN says more than 5,000 people have died as the government seeks to quash 10 months of protests.
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.
Mr Muallem said Arab states had ignored a report from the Arab League observer mission that noted a decrease in the violence in areas it had monitored since the end of December.
In a news conference in Damascus, he signalled no end to the crackdown.
"It is the duty of the Syrian government to take the necessary measures to address the problem of those armed elements who are wreaking havoc throughout Syria," Mr Muallem said.
Fifty-five Gulf Arab monitors were being withdrawn from the mission in Syria, Arab League officials told Reuters - leaving 110 behind.
Earlier, the six members of the Gulf Co-operation Council (GCC) urged the UN Security Council to take "all needed measures... to press Syria to implement the Arab League and the Arab initiative on Syria".
A Kuwaiti newspaper, al-Qabas, reported that the GCC did not want its monitors to be "false witnesses to crimes committed against civilians".
President Assad's supporters in the government and security forces were exploiting the observers to prevent a solution that did not suit them, the paper quoted GCC officials as saying.
Saudi Arabia, the largest of the GCC's six member states, announced it would withdraw its monitors after a meeting of Arab foreign ministers on Sunday, saying Damascus had broken promises on peace initiatives.
The other GCC states are Bahrain, Kuwait, Oman, Qatar and the UAE.
The BBC's Jim Muir in Beirut says the powerful Arab states of the Gulf, with Saudi Arabia and Qatar to the fore, seem to have decided that it is time for President Assad and his regime to go.
They have lost patience with an Arab League observer mission which they believe the Syrians are stringing along to gain time for a "security solution" to the uprising, he says.
The monitoring mission, set up in late December to observe implementation of an Arab League peace plan, had only recently had its mandate extended by the League for another month.
On Sunday, the Arab League put forward a plan for President Assad to hand power to a deputy and for a national unity government to be formed within two months which would include the opposition.
A new constitution would then be drafted and multi-party elections held, according to the plan.
Syria rejected the proposals as "flagrant interference" in its internal affairs.
The Arab League has been pushing for UN Security Council support for its initiatives on Syria.
AFP news agency reported on Tuesday that Qatari Prime Minister Sheikh Hamad bin Jassim al-Thani and the league's Secretary General Nabil al-Arabi had formally requested a meeting with UN head Ban Ki-moon to ask for Security Council backing on Syria.
Security Council action on Syria has been prevented by veto-wielding permanent members Russia and China, who have been reluctant to back action that they say might lead to Libya-style military intervention.