Syrian rebels have seized most of a military academy outside Aleppo after weeks of fighting with regime troops, a network of opposition activists said Saturday, in the latest sign that government forces are losing ground.Rebel brigades have had the sprawling military base in a stranglehold for days.

A rebel commander told CNN in early December that at least 250 government soldiers had defected since the infantry academy came under siege, with most joining the rebel forces.

The Free Syrian Army now has control of most of the base, the opposition Local Coordination Committees for Syria said Saturday.

Heavy clashes were also reported between regime forces and rebel fighters in and around the town of Daraya, near the capital, Damascus.

Regime forces are shelling parts of the city, which has been under siege for weeks, according to the opposition Syrian Observatory for Human Rights.

The Observatory also reported shelling by government security forces of the northeastern towns of Harasta and Erbeen, in the Damascus suburbs, Saturday morning.

There are also clashes between rebel fighters and regime forces along the main highway that passes southern Damascus, it said.

The LCC reported airstrikes by fighter jets on southern neighborhoods of the capital.

The U.N. secretary general's special representative for children and armed conflict, Leila Zerrougui, arrived in the capital Friday to assess the impact of the conflict on the country's children, according to Syria's official news agency, SANA.

Zerrougui will meet Syrian officials, U.N. staffers in the country and members of civil society groups during the four-day visit, it said.

The news agency also reported that government forces had continued their mission Friday to clear areas and neighborhoods in Deir Ezzor, in the country's east, from "mercenary terrorists."

The Syrian government refers to the rebel forces as terrorists.

At least 26 people, including two women, died across the country on Saturday, the LCC reported.

CNN cannot independently confirm government or opposition claims about violence and casualties as Syria has severely restricted access for journalists.

U.S. officials said Friday that Syrian President Bashar al-Assad's control is crumbling at an accelerating pace.

"It's at its lowest point yet," said one senior U.S. official with direct knowledge of the latest assessments. U.S. intelligence believes the decline has accelerated in recent weeks. "The trend is moving more rapidly than it has in the past."

The officials agreed to talk on the condition their names not be used because they were not authorized to discuss the information with the media.

The United States and Germany are sending Patriot missiles and troops to the Turkish border in order to protect their fellow NATO member from potential threats from Syria.

The surface-to-air interceptors would be "dealing with threats that come out of Syria," said U.S. Defense Secretary Leon Panetta Friday. Threats would include Syrian strikes inside Turkey and fighting between the government and rebels that extends into Turkey.

Errant Syrian artillery shells struck the Turkish border town of Akcakale and killed five Turkish civilians in October.

The United States has accused Damascus of launching Scud-type artillery from the capital at rebels in the country's north. One Washington official said missiles came close to the border of Turkey, a staunch U.S. ally.

Syria's government called the accusations "untrue rumors" Friday, according to state news agency SANA. Damascus accused Turkey and its partners of instigating rumors to make the government look bad internationally.

Reports that the Syrian government may be considering the use of chemical weapons have also heightened international concerns in recent days. Syria denies it has such weapons and says it would not use them even if it did.

The Syrian civil war started in March 2011 when a government crackdown on civilian demonstrators morphed into a fight between the regime and rebels.

The conflict has a proxy element, with Sunni countries such as Turkey, Qatar and Saudi Arabia backing the rebels and Shiite Iran backing the Alawite regime. The Alawite faith is an offshoot of Shiite Islam.

More than 40,000 people have died in the war. The United Nations said on Friday that many Syrians will continue to be killed and maimed after the war ends because of deadly explosives placed in residential areas across the country.

 CNN's Arwa Damon and Amir Ahmed contributed to this report.