Panetta, 74, had expected to bow out earlier but his replacement, Chuck Hagel, has had a rough ride so far in the US Senate which has delayed a vote on President Barack Obama's nominee over parts of his otherwise distinguished record.
"It is great to have Panetta back for a last hurrah!" said a senior NATO official, dismissing any suggestion that Hagel's delayed nomination presented a problem.
The United States will in due course nominate another person for SACEUR and the process will go forward as normal, the senior NATO official said.
The two-day defence ministers meeting will focus on capabilities and review progress in 'smart spending' -- making stretched budgets go further as costs rise and governments are under intense pressure to cut spending to balance finances.
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Washington complains that as a result it has ended up shouldering ever more of the burden as its European allies have slashed defence expenditure, to the detriment of their military capabilities.
NATO head Anders Fogh Rasmussen recently warned the 28-member alliance of the seriousness of the budget problem and especially the growing imbalance between Washington and the rest.
"Defence spending among the allies is increasingly uneven, not just between North America and Europe, but also among European allies," Rasmussen said late last month.
Afghanistan and NATO's planned withdrawal in 2014 will be a major talking point at the meeting, after President Obama announced last month that he would cut US troop numbers by just over half this year from 66,000.
The senior NATO official insisted that such talks had to be expected to take some time.