"My message to my Russian colleagues is they too need to recognize the reality of the situation on the ground" in Syria, said Ashton, who was making a two-day visit to Mexico.
Ashton expressed disappointment with Russia's decision on Saturday to veto a UN Security Council resolution -- proposed by Arab and European nations -- which aimed to back an Arab League plan for quelling the violence in Syria.
China also vetoed the measure.
The Arab League plan calls for Syrian President Bashar al-Assad to hand over powers to a deputy so new elections can be held.
The Syrian government's brutal crackdown on opponents of Assad, which began nearly a year ago, has left more than 6,000 people dead, according to rights groups.
The central city of Homs has become a flashpoint, with at least 400 people killed in a relentless six-day onslaught by regime forces, opposition activists said.
US President Barack Obama on Thursday decried what he called "outrageous bloodshed" in Syria.