Adm Mullen met his counterpart amid tensions between the US and Russia
Top US and Russian military leaders have discussed ways to improve relations damaged by Russia's conflict with Georgia.
Adm Michael Mullen, chairman of the US Joint Chiefs of Staff, met his Russian counterpart, Gen Nikolai Makarov, in the Finnish capital, Helsinki.
No details were released following the unannounced talks.
It was the highest-level military meeting between the two sides since the August conflict with Georgia.
An unnamed US military official told AFP news agency: "It is important to have a dialogue with Russia."
He added that there were disagreements and that "Georgia certainly heads the list right now".
The Russia-Georgia conflict followed a sharp deterioration in relations between Washington and Moscow over the past two years.
Russia opposes US plans to install missile defence bases in Poland and the Czech Republic. It was also angered by Nato's expansion into former Soviet republics and US support for the independence of Kosovo.
The fighting in the region began on 7 August when Georgia tried to retake the breakaway region of South Ossetia by force after a series of lower-level clashes with Russian-backed rebels.
Russia launched a counter-attack and the Georgian troops were ejected from both South Ossetia and Abkhazia days later.
Talks between the two sides are set for 18 November after an effort to bring them together failed in Geneva last week.