The fighting around Mannagh airbase near the Turkish border came as foreign ministry officials in Ankara said two Syrian air force generals had defected and crossed the border.
Rebels have been advancing in different areas in northern Syria, capturing several bases in and around the embattled city of Aleppo in recent weeks.
"The fighting did not stop all night around Mannagh airport," said Aleppo-based activist Mohammed Saeed. He added that Syrian military warplanes bombed rebel positions around the camp in an attempt to take the pressure off the base.
The Britain-based Syrian Observatory for Human Rights said rebels reached the edge of the air base late Thursday and that heavy fighting continued into Friday.
The Observatory and Saeed also reported heavy clashes between troops and rebels in an around the Palestinian refugee camp of Handarat in Aleppo, Syria's largest city and commercial center, which opposition fighters have been trying for six months to capture.
Meanwhile, Russia's foreign minister said Moscow has proposed talks with the main Syrian opposition coalition, even though it had previously criticized Western countries for recognizing the group.
Sergey Lavrov told reporters that Russia has contacted the Syrian National Coalition for Opposition and Revolutionary Forces through the Russian Embassy in Egypt and "we expressed readiness to conduct a meeting" with coalition leader Mouaz al-Khatib.
The opposition leader al-Khatib quickly delivered an apparent snub: "If we don't represent the Syrian people why is he inviting us?" He called on Russia to demand that President Bashar Assad step down, "a main condition in any negotiations."
"The Syrian people haven't heard one fair word from Russia to the Syrian people, especially to the children, innocent people and civilians who are killed every day with Russian weapons," al-Khatib said in an interview with Al-Jazeera TV.
Russia has been one of Assad's strongest supporters and used its veto right alongside China at the U.N. Security Council to protect its old ally from international sanctions.
It has increasingly sought to distance itself from the Syrian strongman with top Russian officials, including President Vladimir Putin, making comments that suggest Russia is resigned to Assad losing power.
However, Russia continues to oppose calls for Assad's ouster, and has criticized Western countries for recognizing the opposition coalition, formed in November, as the legitimate representatives of the Syrian people. Moscow says this runs counter to agreements seeking to promote political transition in Syria.
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In Beirut, airport officials said the U.N. envoy for Syria Lakhdar Brahimi left Lebanon to Dubai on his way to Moscow where is expected to discuss the Syria civil war with Russian officials later this week.
In Turkey, foreign ministry officials said Friday that the two defecting generals were "regional Syrian Air Force commanders" and are now in a camp where army defectors stay in the country.
They refused to give the generals' identities or say how they escaped from Syria. They spoke anonymously as they were not authorized to talk to the press.
The defection comes days after the commander of Syria's military police Maj. Gen. Abdul-Aziz Jassem al-Shallal defected to Turkey. Al-Shallal is among the most senior members of Assad's military to defect.
Dozens of Syrian generals have defected since the country's crisis began in March 2011.
In other parts of the country, the Observatory reported fighting and shelling in areas including some neighborhoods of the capital Damascus, its suburbs, the central province of Hama and Homs and the region of Quneitra on the edge of the Israeli-occupied Golan Heights.
The Observatory reported heavy clashes in the southern province of Daraa, mostly in the town of Busra al-Harir, and the Tel Shehab area near the border with Jordan.
Syrian rebels are fighting a 21-month-old revolt against President Bashar Assad's regime. Activists say more than 40,000 people have been killed in the crisis, which began with pro-democracy protests but has morphed into a civil war.
Press; By BASSEM MROUE]
Associated Press Suzan Fraser contributed to this report from Ankara, Turkey.
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