DAMASCUS, Syria – Syria accused neighboring Turkey on Sunday of allowing thousands of Muslim extremists to cross into its territory, as the government and opposition said an explosion killed at least seven and cut off a main road leading south from the capital.

In letters to the U.N. Security Council and Secretary-General Ban Ki-Moon, Syria’s Foreign Ministry said Turkey allowed “thousands of al-Qaida, Takfiri and Wahhabi terrorists” access to the country in order to “kill innocent Syrians, blow up their properties and spread chaos and destruction.”

Syrian authorities blame the anti-government uprising that began in March last year on a foreign conspiracy and accuse Gulf countries Saudi Arabia and Qatar, along with the United States, other Western countries and Turkey, of offering funding and training to the rebels, whom they describe as “terrorists.”

Turkey serves as headquarters for the leaders of the Free Syrian Army rebels and hosts many meetings of the Syrian National Council opposition group. Relations between Turkey and Syria, once strong allies, have been deteriorating since the crisis began last year and Ankara became one of President Bashar Assad’s harshest critics.

Although the conflict has left Syria internationally isolated, Iran has stood by Assad.

On Sunday, the top commander of Iran’s powerful Revolutionary Guard said the elite unit has high-level advisers in Lebanon and Syria but remains undecided on whether to send military reinforcements to help save Assad’s regime.

Sunday’s comments by Gen. Mohammad Ali Jafari were the clearest indication to date of Iran’s direct assistance to its main Arab allies, Assad and Lebanon’s Hezbollah.

It also suggests Tehran is wary about being drawn into a Middle East conflict if outside forces attack Assad, who is locked in a civil war with rebel forces.

Jafari told reporters that Quds force members have been in Syria and Lebanon as advisers for a long time, but he was not more specific.

He said decisions about whether to boost military aid to Syria if attacked would “depend on the circumstances.”

Also Sunday, state-run news agency SANA said rebels detonated a 1,320-pound bomb under the highway near the southern town of Khirbet Ghazaleh. It said the bomb was detonated by remote control and cut the highway that links Damascus with the southern city of Daraa and the Jordanian capital, Amman.

The opposition Local Coordination Committees said seven people were killed and others wounded, although SANA put the toll at eight.

Residents of Daraa province reported Sunday that government troops have been surrounding hamlets in the region. They said the “troops are not allowing people to leave their area, not even to go to school.” One, who identified himself as Abu Wassim, said his elementary school-aged sons could not attend classes on the first day of school in the country.