Syria accused Turkey on Sunday of allowing thousands of Muslim extremists to cross into its territory, just as long-time ally Iran admitted it has its own troops in the war-torn country, providing support to the embattled regime of President Bashar al-Assad.

In letters to the UN Security Council and Secretary-General Ban Ki-Moon, Syria's Foreign Ministry said its northern neighbour has allowed "thousands of al-Qaeda, Takfiri and Wahhabi terrorists" to cross the border in order to "kill innocent Syrians, blow up their properties and spread chaos and destruction."

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

Damascus blames the crisis on a foreign conspiracy and accuses Saudi Arabia and Qatar, along with the U.S, other Western countries and Turkey, of supporting the rebels, whom it describes as "terrorists."

Syria's few remaining allies include Iran, which says it is providing non-military assistance through members of its Revolutionary Guard.

"A number of members of the Qods Force are present in Syria but this does not constitute a military presence," Mohammad Ali Jafari, commander of the elite military unit, said Sunday according to the Iranian news agency ISNA.

Iran is also believed to have supplied weapons to the regime.

U.S. Secretary of State Hillary Clinton earlier this year said Iran is "deeply embedded" and coaching the military in Syria, adding a warning that the conflict could morph into a proxy war in the region.

Also Sunday, Syrian state-run news agency SANA said rebels detonated a 600 kilogram bomb under the highway near the southern town of Khirbet Ghazaleh, killing eight people and wounding 25. The bomb cut the highway that links Damascus with the southern city of Daraa and the Jordanian capital of Amman.

Earlier in the day, government troops captured and cleared the neighbourhood of Midan in the embattled northern city of Aleppo, SANA said, while activists reported that bombardment of rebel-held areas throughout the country claimed the lives of dozens of people.

On Sunday, during an open-air mass in the Lebanese capital, Beirut, Pope Benedict said the Vatican hopes his message of peace is heard in neighbouring Syria, where the conflict that started 18 months ago has claimed more than 20,000 lives.

The waterfront mass for about 300,000 pilgrims was the major public event of the 84-year-old pontiff's three-day visit to Lebanon to press for peace and reconciliation between Christians and Muslims in the Middle East.