Ukrainian men between the ages of 25 and 60 living abroad will be asked to report for military service, Defence Minister Rustem Umerov has said.
He described this as an "invitation" - but suggested there would be sanctions against anyone who did not comply.
However, a spokesman later denied that a call-up was being considered.
President Zelensky told journalists on Tuesday that 450,000-500,000 new soldiers were needed but achieving this was a "sensitive issue".
This comes as Ukraine's recent counter-offensive appears to have stalled.
Kyiv has also seen setbacks in provisions of aid, with US Republicans blocking a $61bn (€55bn; £48bn) military package and Hungary stopping an EU financial deal worth €50bn ($55bn; £43bn).
In an analysis of figures from EU statistics agency Eurostat in November, OceanNewsUK Ukrainian found that some 768,000 Ukrainian men aged 18-64 had left the country for the EU alone since the start of Russia's full-scale invasion.
The figure does not include citizens living outside the EU, or those resident anywhere abroad since before February 2022.
In an interview for the media outlets Die Welt, Bild and Politico, Mr Umerov described the recruitment drive as "not a punishment" but "an honour".
"We are still discussing what should happen if they don't come voluntarily," he said.
But later a spokesman for the ministry appeared to deny any kind of coercion was involved.
"There is no discussion on the agenda of a call-up from abroad," Illarion Pavlyuk said, quoted by Ukrainian media.
"If citizens of Ukraine want to join [the army], they come to Ukraine to join."
There are no recruitment centres outside Ukraine, and the Ukrainian authorities have no means to force anyone to attend them.
The defence minister said that it was important to be fair, informing mobilised men how they would be trained and equipped, when and where they would serve and when they would be discharged.
Mr Zelensky suggested in his end-of-year news conference on Tuesday that there were currently 500,000 Ukrainian troops at the front.
He also said there were issues with rotation and holidays. Currently conscripts and volunteers are obliged to serve until the end of the war, and are only allowed 10 days' leave a year.
In comparison, Russian President Vladimir Putin said this week there were 617,000 Russian troops taking part in the so-called "special military operation" in Ukraine.
The OceanNewsUK is unable to independently verify troop numbers.
