Syrian activists say rebels have captured a military air base in the country's north, their second strategic victory in as many days.

The director of the Britain-based Syrian Observatory for Human Rights Rami Abdul-Rahman says the rebels took control of the al-Jarrah air base in the northern province of Aleppo on Tuesday.

He says that after days of sporadic clashes at the airfield, opposition fighters launched an operation to capture al-Jarrah on Monday and had overrun the facility by Tuesday morning.

The seizure of the airfield comes a day after Syrian rebels captured the country's largest dam in one of their biggest strategic victories since the crisis began two years ago.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

Syrian rebels have scored one of their biggest strategic victories since the country's crisis began two years ago, capturing the nation's largest dam and iconic industrial symbol of the Assad family's four-decade rule.

Rebels led by the al-Qaida-linked militant group Jabhat al-Nusra now control much of the water flow in the country's north and east, eliciting warnings from experts that any mistake in managing the dam may drown wide areas in Syria and Iraq.

A Syrian government official denied that the rebels on Monday captured the dam, saying that "heavy clashes are taking place around it." The official spoke on condition of anonymity in line with regulations. But amateur video released by activists showed gunmen walking around the facility's operations rooms and employees apparently carrying on with their work as usual.

In the capital, Damascus, the rebels kept the battle going mostly in northeastern and southern neighborhoods as the fighting gets closer to the heart of President Bashar Assad's seat of power.

Monday's capture of the al-Furat dam came after rebels seized two smaller dams on the Euphrates River, which flows from Turkey through Syria and into Iraq. Behind al-Furat dam lies Lake Assad, which at 640 square kilometers (247 square miles) is the country's largest water reservoir.

The dam produces 880 megawatts of electricity, a small amount of the country's production. Syria's electricity production relies on plants powered by natural gas and fuel oil.

Still, the capture handed the rebels control over water and electricity supplies for both government-held areas and large swaths of land the opposition has captured over the past 22 months of fighting.

"This is the most important dam in Syria. It is a strategic dam, and Lake Assad is one of the largest artificial lakes in the region," said Rami Abdul-Rahman, who heads the Britain-based Syrian Observatory for Human Rights.

"It supplies many areas around Syria with electricity," Abdul-Rahman said, citing the provinces of Raqqa, Hassaka and Aleppo in the north as well as Deir el-Zour in the east near the Iraqi border.

The dam, constructed in the late 1960s in cooperation with the Soviet Union, is located in a northeastern town once called Tabqa. After the dam was built, the town's name changed to Thawra, Arabic for revolution, to mark the March 8, 1963 coup that brought Assad's ruling Baath party to power.

Early Monday, when the rebels stormed the dam and the town, one of the first things they did was set ablaze a giant statue of the late President Hafez Assad, the current president's father.

"This is one of the biggest projects that have a moral value in Syria's history," said Dubai-based Syrian economist Samir Seifan. "It was the Syrian government's biggest project in the 20th century."

Seifan said that the dam is "a very sensitive plant" and it is very important that technicians and experts keep it running as usual because any mistake could have dangerous consequences.

He added that any mistake could "release massive amounts of water that will drown wide areas including the city of Deir el-Zour as well as cities in Iraq." Seifan added that "any damage will have dangerous consequences on civilians. It supplies hundreds of thousands of hectares with water."

An amateur video released by activists showed rebels walking through large operations rooms as employees went on with their work as usual.

"The al-Furat dam is now in the hands of the Free Syrian Army heroes," says the narrator. "And these are the workers, continuing their work as usual."

The video appeared genuine and corresponded to other Associated Press reporting on the events depicted.

Abdul-Rahman, of the Observatory, said the rebels have told their fighters not to interfere with the work of the dam. He added that the gunmen will leave the dam for employees to run but will keep their checkpoints around the dam.

The rebels now control three dams on the Euphrates. In November, they captured the Tishrin Dam, near the northern town of Manbij. And last week, they took the Baath dam, close to al-Furat.

In Damascus, activists reported clashes and shelling mostly in the northeastern neighborhoods of Jobar and Qaboun as well as the southern parts of the city.

Over the past four days, the rebels brought their fight to within a mile of the heart of the capital, seizing army checkpoints and cutting a key highway.

Syrian TV showed footage from Abbasid Square, a landmark plaza in central Damascus, after sunset Monday to counter activists' claims of fighting only hundreds of meters (yards) away. The footage showed little traffic in the square, and it was dark.

Meanwhile, the Observatory said members of Jabhat al-Nusra blew themselves up in two car bombs outside an intelligence office in the northeastern city of Shadadah, killing at least 14 security agents and wounding many people.

The Observatory said Shadadah has been witnessing heavy clashes between troops and rebels.

The al-Qaida-linked Jabhat al-Nusra, which led the fighting at the dam, has been named by the U.S. government as a terrorist organization. It has proved to be the most effective group among rebels fighting in Syria.

Also in northern Syria, a car bomb exploded at a border crossing with Turkey in Idlib province. Turkey's prime minister, Recep Tayyip Erdogan, said 13 people died in the blast. He didn't specifically say the explosion was caused by a bomb, possibly in deference to an ongoing investigation, but he left little doubt that authorities believed it was the work of assailants.

"The incident is very important in showing to what extent our stance on terror and our sensibility toward Syrian incidents is well-directed," Erdogan said.

The border area between the two countries has seen fierce fighting in the civil war. Tensions have also flared between the Syrian regime and Turkey in the past months after shells fired from Syria landed on the Turkish side.

As a result, Germany, the Netherlands and the United States decided to send two batteries of Patriot air defense missiles each to protect Turkey, their NATO ally.

___

Associated Press writers Chris Torchia in Istanbul and Ezgi Akin in Ankara, Turkey, contributed to this report.