"Teams from the institutions are already on the ground in Athens and work is starting immediately as we speak," European Commission spokeswoman Mina Andreeva told a news briefing.
"Work has started, meaning that the institutions are talking to the Greek authorities."
But a Greek government source insisted that the "negotiations with the technical teams start tomorrow" and not today, in a sign of the continuing difficulties which saw the arrival of the teams pushed back by several days.
The teams - from the European Commission, European Central Bank (ECB), eurozone bailout fund the European Stability Mechanism (ESM) and the International Monetary Fund (IMF) - are now hammering out the final terms for a rescue package worth up to 86 billion euros (USD 94 billion).
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"Progress is (being) made step by step towards a support programme for Greece," Andreeva said.
"More reforms are expected... On the part of the Greek authorities to allow for a swift disbursement under the ESM and this is what is being discussed right now."
Formerly known as the Troika, the creditors were widely loathed in Athens as enforcers of ruinous austerity policies before the government of leftist prime minister Alexis Tsipras kicked them out earlier this year.