She said the deaths were part of a "relentless campaign of airstrikes by the government and its allies", which include Russia. More than 400,000 people have been displaced.
The targets of the attacks were "civilian objects, and it seems highly unlikely, given the persistent pattern of such attacks, that they are all being hit by accident," she said in a statement, adding that the reports were being met with "apparent international indifference", the BBC reported.
Backed by its main ally, Russia, the Syrian government began its offensive against the rebel enclave in northwest Syria the last area of active opposition to President Bashar al-Assad at the end of April, saying it was responding to violations of a ceasefire.
Speaking to reporters, Bachelet hit out at the "failure of leadership by the world's most powerful nations".
The rising death toll in the rebel-held region of Idlib, she said, had been met with a "collective shrug". The conflict had fallen off the international radar while the UN Security Council was paralysed, she added.
She also warned those carrying out the attacks could be charged with war crimes. "Intentional attacks against civil are war crimes, and those who have ordered them or carried them out are criminally responsible for their actions."
Idlib, along with northern Hama and western Aleppo province, is the last opposition stronghold in Syria after eight years of civil war.
It is supposedly covered by a truce brokered in September by Russia and opposition-backer Turkey that spared the 2.7 million civil living there from a major government offensive.
Last week, the UN said more than 350 civil had been killed and 330,000 forced to flee their homes since fighting escalated on April 29.
However, this figure has now been revised, with 103 extra deaths in the last 10 days alone.
The Syrian government, backed by the Russian Air Force, said the increase in attacks was due to what they said were repeated truce violations by jihadists linked to Al Qaeda who dominate the opposition stronghold.
( With inputs from IANS )