"It's a letter from the foreign ministry to the embassy, asking representatives from the embassy to come to Cilacap for a meeting," Ricky Gunawan told AAP.
The Department of Foreign Affairs was contacted for comment.
Indonesia is required to give 72 hours' notice of the executions.
Chan and Sukumaran are in the same cell block as Salami on Nusakambangan, where they and six other drug offenders await execution.
Mr Utomo says based on the experience of the Nigerian embassy in January, when six people were executed, the Saturday meeting means the date is drawing near.
"Last time, when we were asked to gather in the district prosecutor's office, we were then taken to Nusakambangan to tell the convict about the execution time," he said.
"And three days after that, they were executed. I don't know whether other embassies have been notified at the same."
The Attorney-General's spokesman Tony Spontana confirmed reports on Thursday that his office has sent letters to prosecutors advising them to prepare for the executions.
Asked if a date was set he replied: "up to tonight, not yet".
The letters asked prosecutors to make preparations, and were not the notification letters for the convicts themselves, he said.
Lawyers for Chan and Sukumaran have lodged a new appeal in the constitutional court but the attorney-general says the Bali Nine pair have exhausted their legal avenues and he won't recognise the latest action.
A tenth prisoner, Filipina Mary Jane Veloso, is also in line for execution but has yet to be moved to Nusakambangan.
Her lawyers were planning to file for a judicial review on Monday, with new evidence pointing to her being a victim of human trafficking, rather than a drug trafficker.
Her family has arrived in Indonesia from the Philippines and was due to visit her on Friday.
The only Indonesian on death row, Zainal Abidin, still has an application for a judicial review pending in the Supreme Court.Mr Spontana told Indonesian website detik.com the date hadn't been set because of his case.
"We're still waiting for Zainal Abidin," he reportedly said.
"When it's rejected then it will be complete, 10 convicts."
Chan, 31, and Sukumaran, 34, were arrested 10 years ago in a plot to take heroin out of Indonesia, which has so far rejected diplomatic and legal efforts for mercy.
- AAP
