Syrian opposition activists say security forces have used attack helicopters and machine guns to pound targets in Damascus suburbs and in the northern city of Aleppo, in a bid to oust rebels.

Activists say rebels set fire to a police station near Aleppo. They also say clashes and heavy shelling have continued in regions including Homs, Hama and Deir el-Zour as President Bashar al-Assad tries to maintain his grip on power.

The opposition Syrian Observatory for Human Rights tells VOA more than 40 people have been killed in anti-government related unrest across Syria on Wednesday.

U.S. White House spokesman Jay Carney reacted to the violence, saying the reports of President Assad's use of attack helicopters, tanks and other weapons indicates the “depth of depravity” of his regime.

Turkey has closed key border crossings to commercial traffic from Syria, but says they remain open for refugees.

Syrian rebels have captured several crossings. Turkish trucks were burned and looted last week during a battle between opposition and government forces in Bab al-Hawa on the Syrian side of the border.

In another development, Herve Ladsous, the head of U.N. peacekeeping operations, said the Syrian mission continues on a “reduced basis.”

“Reduced in numbers. Reduced in team sites in the province, and it does what it can but, of course, taking into account the security situation, which in many places is extremely delicate.”

On Friday, the U.N. Security Council approved a resolution renewing the observer mission for up to 30 days.