BEIRUT – The head of Syria’s main opposition group in exile called Sunday for international powers to impose a no-fly zone in border areas to protect civilians who are coming under increasingly intense attacks by regime warplanes and helicopters.

The president of the Syrian National Council, Abdelbaset Sieda, said such a move by the international community would show President Bashar Assad’s regime that his opponents around the world are serious.

The Syrian opposition has been calling for a no-fly zone over Syria for months. But Sieda renewed the plea a day after Secretary of State Hillary Rodham Clinton said Washington and Turkey were discussing a range of steps including a no-fly zone over some parts of Syria as the regime increasingly uses its air force to attack rebels.

“There must be special protection,” Sieda said. “The numbers of martyrs are increasing and destruction too. If the country keeps going this way, then we are heading to a catastrophe.”

Russia and China have vetoed attempts to pass tough U.N. Security Council resolutions aimed at Assad’s regime. Last week, the U.N. and Arab League envoy to Syria, Kofi Annan, announced his resignation, following a frustrating six-month effort that failed to achieve even a temporary cease-fire.

Sieda said the no-fly zone should be along borders with Jordan and Turkey, adding that the opposition had called for such a move during last month’s Friends of Syria meeting in Paris attended by world powers.

“Now that Syria’s air force is taking part in bombing cities and towns, there must be protection for the Syrian people. There must be a no-fly zone so that there will be safe havens to refugees,” he said.

Syria’s civil war has spread to almost every province in the country, and the death toll has increased over the past weeks. Activists say more than 20,000 people have been killed since the revolt against Assad’s rule began in March 2011.

Activists reported more clashes Sunday in some Damascus suburbs, the battleground city of Aleppo in the north, central Homs province, and the restive southern town of Daraa.

The deaths of two Syrian journalists Saturday also were reported Sunday.

State-run news agency SANA said one of its reporters, Ali Abbas, was killed at his residence in Damascus. The report blamed an “armed terrorist group” — the regime’s catch-all term for its opponents.

Pan-Arab satellite news channel Al-Arabiya television said that Bara’a Yusuf al-Bushi, a Syrian national and army defector who worked with the station and several other international news organizations, was killed in a bomb attack while covering a story in al-Tal, a suburb in northern Damascus.

The U.K.-based activist group Syrian Observatory for Human Rights said that government forces were able to retake the northern town of Ariha, in the restive Idlib province, that had been under rebels control for months.

The group said that troops backed by tanks entered the town late Saturday and quoted witnesses as saying that many people were detained and some were the victims of “summary executions.”

Also Sunday, SANA said troops killed Wael Mohammed al-Majdalawi, a leader of the Sunni group al-Nusra Front, which has claimed responsibility for deadly attacks over the past months. The attacks included last week’s killing of a prominent Syrian television broadcaster who had been missing since July 19.

In a video released Sunday, Capt. Abdul-Nasser Shumeir, who commands the rebel “Baraa Brigades,” said government troops launched two attacks in an attempt to free 48 Iranians they have been holding for 10 days.

He said Assad’s regime is responsible for the safety of the Iranians, adding that any negotiations for their release should come after government forces lift the siege imposed on rebel-held areas.