Gazprom gave the go-ahead at 10:00 a.m. Moscow time (07:00 a.m. GMT) on Tuesday to resume gas supplies to Europe via Ukraine, but Europe has not received any gas.
"The prime minister informed the European Commission president that Russian gas, supplies of which have been partially resumed, cannot enter Ukraine's gas transit system, which is closed," a Russian government spokesman said.
Barroso expressed a willingness to discuss the problem with the Ukrainian government.
"We have informed the European Commission that we have no real possibility of shipping gas," Gazprom's deputy CEO Alexander Medvedev said during a phone conference with foreign media.
"We get the impression that Ukraine was not planning to comply with its commitments under the agreement signed and open its network for gas transit," Medvedev added.
He also accused Ukraine of failing to provide Russian monitors with access to underground gas storage facilities and control stations, a requirement fixed in a trilateral gas monitoring deal signed on Monday by the European Commission.
The Commission's Energy Spokesman Ferran Tarradellas Espuny said on Tuesday observers had still not received full access to monitoring stations in both Moscow and Kiev, and urged the resumption of gas supplies to Europe as soon as possible.
A source in the international monitoring commission said Russian gas intended for transit to Europe could not enter Ukraine's pipeline network because the taps were turned off.
Another source close to the Ukrainian energy company Naftogaz said that Gazprom had failed to provide sufficient volumes of gas to allow transit to Europe via Ukraine's network, Ukraine's UNIAN news agency said.
Naftogaz spokesman Valentin Zemlyanskiy cited Russia's "unacceptable transit conditions" as the reason for blocking its natural gas supplies.
Earlier in the day, Ukrainian President Viktor Yushchenko ordered controls on Ukraine's gas transportation system to be tightened following the resumption of gas transits to Europe from Russia. He also sent a corresponding letter to the country's interior ministry, security council, and emergencies ministry.
"Due to the resumption of natural gas supplies to Western Europe by the Russian Federation, I am requesting you, within your competence and the existing legislation, to take measures to prevent unexpected situations arising on the Ukrainian gas transportation network," the letter said.
The EU gets some 25% of its gas from Russia - 80% of which passes through Ukraine.