LATEST: British Foreign Secretary Boris Johnson has denounced a chemical weapons attack in Syria - which it is reported has claimed the lives of 58 people - as a "war crime" and called for those responsible to be brought to justice.

It's raised fears of more deadly violence between groups battling President Bashar Assad's troops.

Syria's rebel-held Idlib province came under heavy bombardment on Sunday as rebels and pro-government forces battled for control of the nearby city of Aleppo, activists said.

Anti-government activists say air strikes have killed at least 12 people at a market and other targets in the north-western Syrian city of Idlib.