Activists: Rebels pound military base

• Peacekeeping chief to discuss the mission with the U.N. Security Council
• Rebel fighters and regime forces battle for control of Aleppo
• Bashar al-Assad urges his armed forces to restore stability to the nation
• His message gives no indication of his whereabouts

(CNN) -- Syrian rebels attacked a military airport near the commercial capital Thursday morning, opposition activists said, a day after President Bashar al-Assad urged his forces to push on in the raging battle.

"Menagh military airport sustained shelling on Thursday morning from tanks taken by rebels in previous operations," the Syrian Observatory for Human Rights said of the base north of Aleppo.

Rebel fighters have been battling regime forces for control of Aleppo, the country's most populous city and commercial hub.

The airport attack comes after al-Assad exhorted his armed forces Wednesday to restore stability to the nation amid a growing armed resistance and mounting deaths.

"The enemy is among us and is using inside agents to destabilize the country and the security of its citizens," the president said, according to the state-run Syrian Arab News Agency. The message did not give any indication of his whereabouts.

Two activists reported that communications in Aleppo are largely cut -- including cellular networks, landlines and Internet service. "There are whole neighborhoods where we do not know what's happening," said an activist who did not want to be named for safety reasons.

Seventeen people were killed Wednesday in the city, the opposition Local Coordination Committees of Syria said. They were among 180 people killed nationwide that day, including 50 of whom were victims of a massacre in Jdeidet Artouz in Damascus suburbs, the group said.

CNN cannot independently confirm reports of violence in Syria because the government has limited access by international journalists.

Rebels said the situation in Aleppo is playing to their favor.

"There is no way for Assad's ground troops or tanks to enter certain areas by land due to our strategic control of the highways," said Col. Malek-Al Kurdi, deputy head of the Free Syrian Army in Aleppo.

The U.N. Supervision Mission in Syria, which is monitoring a failed peace plan, reported increased fighting in the city over the past few days.

"Our observers are reporting exchange of fire, shelling and explosions in addition to use of helicopters, tanks, heavy machine guns and artillery shelling," said Sausan Ghosheh, a spokeswoman for the mission.

She said observers noted the opposition forces are equipped with heavy weaponry, including tanks.

Syria's conflict has erupted in Damascus and Aleppo, both major prizes for the warring sides as the battle intensifies and the international community scrambles to find a resolution.

On Thursday, Under-Secretary-General for Peacekeeping Operations Herve Ladsous is expected to report to the U.N. Security Council on the supervision mission, whose mandate was renewed last month for 30 days.

The Syrian crisis started when al-Assad's security forces launched a violent crackdown on peaceful protests in March 2011. That clampdown spurred a nationwide uprising as armed rebels, including military defectors and other fighters, battled under the Free Syrian Army.

The conflict has claimed roughly 17,000 lives, U.N. Secretary-General Ban Ki-moon said last week. Opposition activists put the toll at more than 20,000.

CNN's Salma Abdelaziz and Faith Karimi contributed to this report.