Syrian opposition activists say the government has sent troop reinforcements to the city of Aleppo, where fierce fighting between rebels and the military continued for a fifth day.

The Britain-based Syrian Observatory for Human Rights said clashes were taking place Wednesday in several Aleppo neighborhoods, and that Syrian forces were using helicopters.

The Observatory says at least eight people were killed by shelling and gunfire in Aleppo province, Hama and Idlib. Activists say government forces on Wednesday also shelled the Damascus suburb of al-Tel.

Meanwhile officials in neighboring Turkey said border crossings between the two countries would close Wednesday because of worsening security conditions.

Syrian rebels have captured several crossings. Turkish trucks were burned and looted last week during a battle between opposition and government forces in Bab al-Hawa on the Syrian side of the border.

Russian Foreign Minister Sergei Lavrov on Wednesday accused the United States of “justifying terrorism” by not condemning a bombing last week in Damascus that killed several top security officials.

Russia also said that new European Union sanctions calling for members to inspect suspicious sea and air cargo headed for Syria amounted to a “blockade.” The Foreign Ministry said in a statement that the measure is “counterproductive” and goes against the spirit of U.N. envoy Kofi Annan's peace plan.