Kerry said at a meeting of NATO foreign ministers in the Turkish city of Antalya that now is a "critical moment for action by Russia and separatists to live up to the Minsk agreement" signed in the Belarus capital earlier this year.
He added that it was also "critical" for observers from the Organization for Security and Cooperation in Europe (OSCE) to be allowed into the conflict areas to monitor the truce.
The intensity of the fighting in eastern Ukraine controlled by pro-Russia separatists has declined since the Minsk deal but deadly incidents remain frequent.
Kerry had met Putin on Tuesday for the hightest level US visit to Russia since the Ukraine conflict erupted in 2013, in a possible sign of a cautious thaw between the two sides.
Kerry said the United States would prefer not to keep sanctions in place against Russia but would keep the measures for as long as required to ensure peace.
He said there was "unanimity" on the part of NATO members on the importance of the Minsk agreement being fulfilled by Russia.
"This is an enormous moment of opportunity for the conflict... to find a path of certainty and resolution," said Kerry, expressing hope that Putin would fulfil his obligations.
"Actions speak louder than words," said Stoltenberg, adding that Russia's "more aggressive behaviour" was a challenge for NATO.
"Now is the time to act... there is urgency when it comes to fulfilling the Minsk agreement," he said.
The NATO ministers are meeting for two days in the Turkish southern resort city. However, Kerry is leaving early to attend a summit of Gulf leaders hosted by President Barack Obama.
The West accuses Russia of arming separatists in eastern Ukraine and even sending its own troops across the border. Russia denies the charges.