Syria conflict: Intense fighting in north threatens truce
Fighting in northern Syria has intensified, posing the biggest threat yet to the cessation of hostilities that came into force six weeks ago.
Government forces, backed by Russian troops, have pounded rebel-held parts of Aleppo, targeting supply lines.
Meanwhile, fighting further north between the self-styled Islamic State (IS) group and other rebels is said to have displaced 30,000 people.
The violence is overshadowing the resumption of peace talks in Geneva.
Earlier, the main Syrian opposition group said it was ready to talk to members of the government who had not been involved in the killings - but not with President Assad himself.
Government delegates arrive in Geneva on Friday. They have said Mr Assad's fate remains non-negotiable, leaving little hope of compromise or concession.
Syrian government war planes, helicopter gunships and artillery bombarded rebel-held parts of Aleppo on Thursday, activists said.
Fighting was said to be fiercest to the north of the city, along a key route out of the rebel-held area.
The government offensive on Aleppo has been under way for the last two weeks, supported by Russian air strikes, Iranian Revolutionary Guards personnel and fighters from Lebanon's Hezbollah movement.
Further north, militants from the so-called Islamic State were battling to regain momentum by seizing several villages close to the Turkish border.
Human Rights Watch said the fighting had displaced at least 30,000 people in just 48 hours and accused Turkish guards of shooting at those who tried to cross the border into Turkey to seek protection.
The group called on Turkey to open its borders to those fleeing, saying some families had been forced to "dig ditches" to try and hide from IS and other militants.
The spike in violence has alarmed the international community who helped broker the 27 February cessation of hostilities.
One senior US administration official said all sides should follow through on their commitment "to refrain from any further destabilising actions."
European Union foreign policy chief Federica Mogherini called for the "respect, consolidation, and expansion" of the truce.
Russian President Vladimir Putin - an ally of President Assad - said he was watching the situation in Aleppo closely and voiced hope that the peace talks would lead to a new Syrian constitution and an early election.
UN special envoy Staffan de Mistura, who is mediating the talks in Geneva, on Wednesday warned of an urgent need for the government and opposition to take steps towards a political transition.
He said he wanted this - the third round of "proximity" talks since January - to be "quite concrete" with both sides agreeing on a political process that a UN Security Council resolution passed in December, which envisages the formation of a transitional government, a new constitution and elections.
The veteran Swedish-Italian diplomat also stressed that it was very important that the cessation of hostilities continued and that humanitarian aid was allowed to reach every Syrian.