Rebels in Syria have captured 32 soldiers and pro-government gunmen near the northern city of Aleppo, where fighting is raging as the two sides try to grab new territory ahead of a possible truce, activists said Thursday.

The fighting comes as U.N. envoy Staffan de Mistura is trying to broker a truce for war-ravaged Aleppo, Syria's largest city. On Tuesday he said in New York that Syrian President Bashar Assad has expressed willingness to suspend bombing of Aleppo for six weeks.

The Britain-based Syrian Observatory for Human Rights and Aleppo-based activist Ahmad Hamed said the troops were seized in the village of Ratyan after it was retaken by the rebels on Wednesday. More than 170 fighters have been killed from both sides in the clashes, according to the Observatory.

An amateur video posted online showed some 28 men in military uniforms sitting on the floor inside a room. The men, who included at least two wounded, were then asked to stand up and leave the room one by one under the watch of rebels.

The video appeared genuine and corresponded to other Associated Press reporting of the capture and fighting in the area north of Aleppo.

The Observatory said rebels also recaptured the village of Hardatnein Thursday, two days after it was taken by government forces. The Observatory and Hamed said intense clashes are taking place outside a third village, Bashkoy, which was also taken by the government on Tuesday.

State-run Syrian TV reported that troops are still advancing in the north, adding that Syrian opposition fighters are coming from Turkey to take part in the battles near Aleppo.

The Observatory said 90 troops and pro-government gunmen, in addition to 81 rebels, have been killed since the fighting began on Tuesday.

In Geneva, the UN High Commissioner for Human Rights Zeid Raad al-Hussein urged Syrian authorities Thursday to release all those held by government forces and pro-government groups. Some detainees have been held for years.

The High Commissioner estimated the number of people in Syria who have been held at some point or other in government and intelligence detention facilities to range from tens of thousands to hundreds of thousands since the crisis began in March 2011.

"I urge the Syrian Government to immediately release all those who have been jailed for peaceful expression of their views and to ensure that all those detained are accorded their full due process rights," Zeid said.