Syrian warplanes and artillery pounded opposition strongholds in Damascus and its suburbs Tuesday, as rebels in the northern city of Aleppo launched an operation to try to free hundreds of political detainees from the city's central prison, activists said.

The fighting has escalated across Syria in recent weeks, particularly in Aleppo and Damascus, the country's largest cities, as the rebels and President Bashar Assad's regime try to gain the upper hand in the 2-year-old conflict.

The Britain-based Syrian Observatory for Human Rights said some 6,000 people were killed nationwide in March alone, making it the deadliest month since the uprising began.

Fighting has also intensified in the southern provinces of Daraa and Quneitra, across from Jordan and the frontier with Israel.

On Tuesday, the Israeli military said a mortar shell landed on its side of the frontier in the Golan Heights area, but it was not clear if the fire was intentional. The incident came after Israeli Defense Minister Moshe Yaalon visited the region Tuesday and warned that Israel would respond to any attack from Syria.

The escalation in the southern Quneitra region along the cease-fire line separating Syria from the Israeli-occupied Golan Heights accented concerns that Islamic extremists among the rebel forces could take over the front line opposite Israeli troops.

Damascus has become a key battleground in the civil war. From their strongholds in the suburbs, rebel fighters are trying to slowly push their way into the heart of the capital. Assad has deployed his most loyal and best equipped troops there, trying to insulate it from the violence.

The Observatory said government forces on Tuesday shelled the northern Damascus neighborhoods of Jobar, Barzeh and Qaboun. It also reported an air raid on the suburb of Mleiha.

Maath al-Shami, an activist based in the suburb of eastern Ghouta, said there were several airstrikes in the area, which includes Mleiha. "Fighting is taking place on all fronts," he said, referring to clashes east of Damascus.

"Drones fly over eastern Ghouta then warplanes come and bomb the area," al-Shami said, using his activist name because he feared government retribution.

State-run TV said rebels fired a mortar shell on the Damascus suburb of Muqailabiyeh, killing four people, including two children. It claimed troops killed scores of gunmen throughout the country.

The Observatory also reported that five people were killed, including a woman and an Iraqi national, when a barrage of mortar shells exploded in an area of the suburb of Jaramana, just few kilometers (miles) southeast of Damascus. The town is overwhelmingly pro-regime.

In Aleppo, rebels launched an attack dubbed "Freeing the Prisoners" that aims to eventually free detainees held in the city's central prison, the Aleppo Media Center activist group reported.

Aleppo-based activist Mohammed Saeed said the operation is a three-pronged attack targeting at the Kindi Hospital, Ghondol square and the central prison. He said the hospital has been turned into a military compound recently.

"The aim of the offensive is to strengthen the siege on the central prison and demand the release of political prisoners," Saeed said via Skype, adding that the goal is to free hundreds of political prisoners inside.

He added that if the rebels manage to take the three areas, they can cut supply lines for government troops stationed in the city.

Aleppo, Syria's largest city and a former commercial hub, has been a key battleground in the country's civil war since rebels launched an offensive there in July, seizing several districts before the fighting largely settled into a bloody stalemate that has left the city carved up into government- and rebel-held zones.

In an attempt to combat rising crime and kidnapping for ransom as a result of the civil war, Assad issued a decree Tuesday threatening life in prison at hard labor for any person who abducts someone for political or sectarian reasons or for ransom, the state news agency said.

It said if the kidnapper kills, rapes or permanently disables the captive, then the abductor will receive the death sentence.

In neighboring Lebanon, security officials said tribal leaders in the northern region of Wadi Khaled were expected to meet to discuss freeing eight Syrians who were kidnapped while on their way to Syria on Monday.

Lebanon's state-run National News Agency reported that the eight were kidnapped by relatives of a Lebanese man who has been held by Syrian authorities for about a year. Local media reported that the eight were Alawites -- members of Assad's minority sect.

Lebanon is sharply divided between supporters and opponents of Assad and along sectarian lines. Violence from Syria has spilled over into deadly street fighting and kidnappings on several occasions.

Also Tuesday, Syrian state-run Al-Ikhbariya TV said the government has decided to exempt fuel and diesel imports from Iran from tax and customs charges until June 30. Syria has been suffering major shortage of fuel after repeated attacks on oil pipelines.

In Geneva, the U.N.'s World Food Program said unknown assailants in Syria have carried out at least 20 attacks on its food trucks, warehouses and cars since emergency operations were launched in December 2011.

WFP spokeswoman Elisabeth Byrs says no one has been hurt in the attacks, but she could not provide details on locations or the assailants.

Byrs told reporters Tuesday in Geneva that the attacks show "it's becoming more and more difficult with this growing violence to reach the people who are in need of assistance."

The U.N.'s food agency says 2.5 million people in Syria and almost 1 million refugees in neighboring countries need its help, costing $19 million a week.

A U.N. panel estimated in mid-February that at least 70,000 people have died in Syria's civil war.