BEIRUT -- The head of the Arab League warned Friday that Syria may be sliding toward civil war, as security forces fired on thousands of people who poured into the streets in support of army defectors who switched sides to try to topple President Bashar Assad.

Also Friday, an activist group said two foreign journalists and a translator were arrested in the Syrian capital, Damascus. The group, the Local Coordination Committees, had no further details.

The government has barred access to the country by most foreign media, except on a limited number of escorted trips.

Over the course of the 10-month-old uprising, much of the bloodshed has been from security forces firing on unarmed protesters. But in recent months breakaway soldiers have been attacking the Syrian military, and some opposition members have taken up arms against the regime, adding to the violence.

Despite that, Assad appears to maintain a firm grip on power in the face of growing international pressure to halt his crackdown and step down.

The Arab League chief, Nabil Elaraby, told The Associated Press that Assad's regime was either not complying or only partially complying with an Arab League plan that Syria signed last month to end its crackdown.

"We are very concerned because there were certain commitments that were not complied with," he said in Cairo, where the League is based. "If this continues, it may turn into civil war."

The U.N. estimates more than 5,000 people have been killed since the uprising began in March.

The British-based Syrian Observatory for Human Rights said security forces killed one protester in the central city of Hama and another in the town of Ariha in the northwestern province of Idlib, where more than 20,000 people were demonstrating Friday.

The Observatory reported violence in the southern province of Daraa, the eastern region of Deir el-Zour and the central province of Homs, all centers of frequent protests.

A video posted online by activists showed dozens of people marching in the Damascus neighborhood of Midan, chanting "Freedom forever, despite you Assad!" Midan, where there have been frequent anti-regime protests, was hit by a suicide attack last Friday that killed 26 people.

It wasn't clear who was behind that attack; the government blamed "terrorists" while the opposition suggested the regime orchestrated the blast to tarnish the uprising.

Another video posted Friday showed what appeared to be an armored personnel carrier on fire. The narrator said army defectors attacked the vehicle with a rocket-propelled grenade.

The Arab League plan calls for removing Syrian forces and heavy weapons from city streets, starting talks with opposition leaders and allowing human rights workers and journalists into the country.

An Arab League team of observers began work in Syria on Dec. 27 to offer an outside view of whether the government is abiding by its agreement to end the military crackdown on dissent.

The mission has been plagued by problems, including accusations that the Syrian government is interfering with the team's work. This week, one of the observers resigned and told the pan-Arab TV channel Al-Jazeera that the monitor mission was a "farce" because of Syrian government control.

Adnan al-Khudeir, head of the Cairo operations room to which the monitors report, told reporters Thursday that two more observers, from Algeria and Sudan, would be returning to their home countries. He did not identify them but said the Algerian gave health reasons and the Sudanese cited personal reasons.