Syrian troops and rebels have clashed in the commercial capital, Aleppo, near the city's government-held airport, as battles intensify ahead of a visit by the new international peace envoy to Syria.

The Britain-based Syrian Observatory for Human Rights and other activist groups said the heavy fighting erupted at dawn Wednesday in the Nayrab area, about five kilometers from Aleppo International Airport.

The facility, which includes a military base, is widely used by the government to bomb rebel-held areas. Over the past several weeks, rebels have been attacking military airfields in an attempt to prevent them from being used for launching air strikes, while commercial facilities have been left alone.

Diplomatic efforts have so far failed to stop the bloodshed in Syria, but the new U.N.-Arab League envoy, Lakhdar Brahimi, plans to travel to the country this week in a bid to revive them.

U.N. Secretary-General Ban Ki-moon said Brahimi will sit down with President Bashar al-Assad when he makes his first official visit to Syria, although the date has not been set.

Brahimi replaces former envoy Kofi Annan, who failed to reach a cease-fire in Syria and open talks on a transitional government.

U.N. officials say the fighting in Syria has killed about 20,000 people, nearly all of them civilians, and driven more than one million from their homes.

More than 250,000 Syrians have fled to Jordan, Iraq, Lebanon and Turkey. The U.N refugee agency calls the humanitarian problems caused by the war its biggest crisis.