DAMASCUS, Syria (AP) — Syria accused neighboring Turkey Sunday of allowing thousands of Muslim extremists to cross into its territories, as the government and opposition said an explosion killed at least seven and cut off a main road to 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 uprising that began in March last year on a foreign conspiracy and accuse Gulf countries Saudi Arabia and Qatar, along with the U.S, other Western countries and Turkey, of offering funding and training to the rebels.

Turkey serves as headquarters for the leaders of 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 President Bashar Assad’s harshest critics.

Also Sunday, state-run news agency SANA said rebels detonated a 600 kilogram (1,320 pound) bomb under the highway near the southern town of Khirbet Ghazaleh, killing eight people and wounding 25. 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 of Amman.

SANA added that nine cars and two buses carrying state employees were damaged in the blast. The opposition Local Coordination Committees said seven people were killed and others wounded in the blast that it said damaged a bus.

Earlier in the day, government troops captured and cleared the neighborhood 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.

SANA said troops also killed dozens of rebels in other parts of Aleppo, the country’s largest city and commercial center. The LCC and the Britain-based Syrian Observatory for Human Rights said many of the dead in Aleppo were killed in air raids on the rebel-held neighborhood of Shaar.

The fight for Aleppo, a city of 3 million that was once a bastion of support for Assad, is critical for both the regime and the opposition. Its fall would give the opposition a major strategic victory with a stronghold in the north near the Turkish border. A rebel defeat, at the very least, would buy Assad more time.

The Observatory also said troops shelled the Damascus neighborhood of Hajar Aswad, an area that has witnessed anti-government activities since the early weeks of the uprising.

Sunday’s countrywide death toll reached over 50 people, the Observatory added. It was the same day that the school year began for some five million students.

The Syrian uprising began with mostly peaceful protests in a number of the country’s impoverished provinces. As security forces violently suppressed them, the protest grew and escalated into an increasingly armed insurrection. Activists say at least 23,000 people have been killed in the past 18 months.

On Sunday, the new international envoy tasked with ending the civil war, Lakhdar Brahimi, left Syria, ending a four-day visit during which he met with Assad and other officials, his office said in a statement. Brahimi summed up his first foray to Damascus Saturday with a startling and frank admission that he still has no plan for stopping the bloodshed which he warned could threaten world peace.

In Damascus, some 24 political parties and groups who describe themselves as opposition organizations gathered in an effort to seek national reconciliation and end violence in the country.

The participants, who included Deputy Prime Minister Qadri Jamil — head the opposition party Front of Change and Liberation, said in a statement that they aim to launch a pure Syrian dialogue and call for a national conference to be attended by all opposition groups from inside and outside the country.