"Let's be clear about what this agreement does: It permits the assault on Aleppo to continue for another week. It requires opposition groups to stop fighting but it allows Russia to continue bombing terrorists which it insists is everyone, including civilians," said McCain, a leading member of the opposition Republicans and head of the Senate Armed Forces Committee.
"If Russia or the Assad regime violates this agreement, what are the consequences? I don't see any," he told an audience at the Munich Security Conference.
Critics have said the deal is hobbled by the fact it does not include "terrorist" groups such as the Islamic State group and the Al-Qaeda affiliate Al-Nusra, leaving room for Russia to continue attacks by claiming it is targeting jihadists.
The agreement followed a major offensive by Syrian government forces, backed by heavy Russian bombing and Iranian troops, on the rebel stronghold of Aleppo.
McCain said it was "no accident" that Russian President Vladimir Putin had chosen this moment for a deal.
"This is diplomacy in the service of military aggression and it is working because we are letting it.
