The letter accused Washington "of actual involvement in bloody events in Syria" and asked the 22-member League to "condemn the involvement and to do what is necessary to end it," the group said in a statement.
It did not provide any details on the charges of US involvement in the Syrian bloodshed.
But Syria has in the past accused the US ambassador to Damascus, Robert Ford, of inciting violence by visiting protest hubs, before Washington recalled him last month following "credible threats against his safety in Syria."
In the letter, Syria sought Arab assistance "to provide the appropriate atmosphere to implement the agreement," said the statement.
Arab League deputy chief Ahmed Ben Hilli told AFP that Syria had sent a letter detailing the steps it took towards carrying out the plan, but he refused to elaborate.
The League has called an emergency meeting in Cairo on Saturday about Syria's failure to implement its roadmap, which calls on President Bashar al-Assad to open talks with the opposition and withdraw tanks from the streets.
Elsewhere on the diplomatic front, British Foreign Secretary William Hague called for greater pressure on the regime, rather than military intervention, to end the violent repression in Syria.
"Of course, the UK would like to be able to pass a resolution at the UN Security Council bringing the condemnation of the world on the use of force against civilians by the Syrian regime," he said.
On the ground, five civilians were killed as heavy artillery clashes erupted between regime forces and presumed army defectors in the central city of Homs, a human rights group reported.
It was the fifth day of a "brutal siege on the brave city," said the Syrian Observatory for Human Rights.
The opposition Syrian National Council appealed to the international community to send "Arab and international observers, instantly, to the city of Homs to oversee the situation on the ground, and prevent the regime from continuing to commit brutal massacres."
The SNC urged the United Nations, the Organisation of Islamic Cooperation and the Arab League to act "to stop the massacre committed by the regime" in Homs, which it declared a "humanitarian disaster area."
It also called for the evacuation of civilians from "areas that are under shelling and destruction" in the industrial city, a tinder box of sectarian tensions that risk escalating into a civil war.
Assad's forces, it said, had "launched a large-scale attack" overnight on parts of Homs, and that "indiscriminate slaughter is being committed by the regime's militias."
The army was "using heavy artillery, rocket launchers, and warplanes to bomb populated residential neighborhoods" in Homs.
Homs is the only major city to remain outside the regime's control after military operations in Hama, Deir Ezzor in northeastern Syria and the coastal cities of Latakia and Banias reined in the dissent.
The Syrian Observatory for Human Rights said five civilians were killed during the clashes in Homs.
"Shooting could be heard in Homs where neighbourhoods came under heavy machinegun fire at dawn," the Britain-based watchdog said, adding "more than 40 explosions were heard."
Residents there saw a truck "filled with corpses."
In the Hula area of Homs province, an eight-year-old girl was killed and a woman wounded when forces "fired indiscriminately," said the Observatory, adding another two civilians were killed at Kfarzita, in Hama province to the north.
Soldiers and gunmen believed to be dissidents also clashed in Khan Sheikhun in Idlib province, near Turkey, but there was no report of casualties.
Soldiers also searched cars for people "wanted" by the regime, it said.
In Damascus province, a 63-year-old man died of his wounds after being shot by security forces the previous day, the rights group said.
In Banias, worshippers leaving Al-Radwan mosque staged a rally calling for the "fall of the regime" and the "execution of the president."
Forces responded by raiding the homes surrounding the mosque.
The latest clashes bring to at least 80 the number of people killed since Syria signed the Arab peace plan on Wednesday last week.
Regime forces reportedly killed at least 19 demonstrators on Sunday after prayers on Eid al-Adha, one of Islam's holiest days.
The United Nations estimates more than 3,000 people have been killed across Syria in the security crackdown since pro-reform protests erupted in mid-March.
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