Damascus has accused Washington of interfering in the work of the Arab League after a US official travelled to Cairo for talks with the bloc about the protest crackdown in Syria.
"The United States is one of the parties which is seeking to rekindle violence by its mobilisation and incitement [to violence]," Jihad Makdisi, foreign ministry spokesman, said in a statement on Wednesaday.
"The US... statements are a gross interference in the work of the Arab League, and an unjustified attempt to internationalise" the issue of Syria, he said.
The US State Department said on Tuesday that Jeffrey Feltman, the assistant secretary of state for Near East Affairs, was to travel to Cairo for consultations with the Arab League about Syria.
"We have made clear that if the Arab League initiative is not implemented, the international community will have to consider new measures to compel a halt to the regime's violence against its own citizens,'' spokesman Jay Carney said on Tuesday.
There are about 100 Arab League monitors in Syria, dispatched to verify the government's compliance with a plan to stop its crackdown on dissent. However, activists and the regional body itself has said security forces are still killing anti-government protesters despite the observers' presence.
The UN said in December that more than 5,000 people have been killed in the continuing crackdown. Since that report, opposition activists say hundreds more have been killed. Syrian authorities say armed groups have killed about 2,000 security forces personnel.
Colonel Riad al-Asaad, the head of the Free Syrian Army (FSA), has threatened to step up attacks on government forces, saying he was frustrated with Arab League monitors' lack of progress in ending a government crackdown on protests.
Al-Asaad said he is waiting for the regional bloc's report on its first week before deciding whether to make a "transformative shift" that he said would mark a major escalation against the security forces.
In-depth coverage of escalating violence across Syria
"If we feel they are still not serious in a few days, or at most within a week, we will take a decision which will surprise the regime and the whole world," he told the Reuters news agency on Tuesday by telephone from his safe haven in southern Turkey.
"Since they [the monitors] entered, we had many more martyrs."
Activists put the death toll at more than 400 people since Dec. 21, with reports from Local Coordination Committees that 21 people were killed in clashes across the country on Wednesday.
Al-Asaad defected from the Syrian air force in July. The strength of his forces is unknown, but according to media estimates, the umbrella organisation has more than 10,000 men in its ranks.
League claims 'noticeable progress'
The Arab League said it would hold a meeting to look into the first report by the head of the monitoring mission. Originally scheduled for Saturday, the meeting has been postponed to Sunday due to the Orthodox Christmas.
Adnan al-Khudeir, the Arab League official who heads the operations room that the monitors report to, said the meeting would be chaired by Hamad bin Jassim Al Thani, the Qatari prime minister.
"There is noticeable progress," al-Khudeir said, referring to the reports he received so far. "It is hard to make a judgement on the mission of the monitors because they are still in the beginning ... We can't tell if they failed or succeeded right now."
Nabil Elaraby, the Arab League secretary-general, said on Monday that killings were continuing in Syria, but that the military had withdrawn tanks and artillery from residential areas and was on the outskirts of the cities.
"Yes, killings continue," he said. "The objective is for us to wake up in the morning and hear that no one is killed. The mission's philosophy is to protect civilians, so if one is killed, then our mission is incomplete.''
Activists have rejected claims that the military has withdrawn from the cities, posting videos online of soldiers close to a group of Arab League observers on Tuesday in the flashpoint city of Homs.
The Arab League mission has come under harsh criticism by opponents of President Bashar al-Assad's government who say some observers lack experience.
Its chief, Lieutenant-General Mohamed Ahmed Mustafa al-Dabi, raised opposition concerns because he served in critical security positions under Omar al-Bashir, the Sudanese president who is wanted on an international arrest warrant for crimes against humanity in Darfur.
The Local Co-ordination Committees, an umbrella group of activists, says the observer mission is witnessing mainly government-staged events, and they move about the country only with the full knowledge of the government.
However, video clips posted by activists appear to show observers moving around in protesting crowds, on some occasions within hearing distance of gunfire.