A Russia-backed regime offensive on the last major opposition stronghold in northwestern Syria has displaced nearly 700,000 people since December, the U.N. told Agence France-Presse (AFP) on Monday.

The violence in the provinces of Idlib and Aleppo has displaced 689,000 people, said David Swanson, spokesperson for the U.N.'s humanitarian coordination office, OCHA.

Ankara and Moscow agreed last year in Sochi to stop acts of aggression and turn Idlib into a de-escalation zone, which was to be monitored by 12 Turkish observation points.

However, the regime, Iran-backed militia groups and Russia have consistently violated the cease-fire, launching frequent attacks inside the de-escalation zone. Three observation points in the region – point seven, eight and nine – are currently under siege by the regime forces.

More than 1.5 million Syrians have moved near the Turkish border due to intense attacks over the past year.

Turkey remains the country with the most refugees in the world, hosting more than 3.7 million migrants since the Syrian Civil War broke out in 2011.