Speaking in Kiev, Gen Breedlove said Russian military assets being installed in Crimea would have an effect on "almost the entire Black Sea".
Mr Breedlove is in Ukraine for high-level talks with Ukrainian leaders.
Russia annexed the Crimean Peninsula from Ukraine in March 2014.
Russia's defence ministry said on Wednesday that it had deployed a batch of 14 military jets to Crimea, as part of a squadron of 30 that will be stationed on the peninsula.
An initial batch of fighter jets were flown to Crimea's Belbek air base "from military air bases in Krasnodar Territory," Russian agency Interfax reported.
Gen Breedlove had said earlier on Tuesday that a large number of Russian troops were also active inside Ukraine, training and advising separatist rebels.
Russia has continued to deny allegations from western countries that it played any direct role in the conflict in Ukraine, which has claimed more than 4,317 lives.
President Vladimir Putin said that Russia "poses no threat to anyone" and would "resist efforts to draw it into geopolitical intrigue," Russia's Tass news agency reported on Wednesday.
Meanwhile, German Chancellor Angela Merkel launched a strong attack on Russia's actions against Ukraine whilst addressing a session of parliament in Berlin.
"Nothing justifies or excuses the annexation of Crimea by Russia... Nothing justifies the direct or indirect participation of Russia in the fighting in Donetsk and Luhansk," she said, speaking in the Bundestag.
"Russia is calling into question Europe's peaceful order and it is trampling on international law."
She added that the possibility of a lasting ceasefire in eastern Ukraine was unlikely and therefore continued economic sanctions on Russia remained "unavoidable".
The US and the EU have placed sanctions on Russia for its alleged involvement in the Ukrainian crisis.
Human cost of conflict in east Ukraine
4,317 deaths since April - 957 of them since the 5 September ceasefire, and 9,921 people wounded
466,829 internally displaced people within Ukraine
454,339 refugees living abroad, 387,355 of them in Russia
UN data from 18 November
Ukraine's year of chaotic events
Nato's top military commander, Gen Philip Breedlove, has warned that Russian "militarisation" of the annexed Crimea Peninsula could be used to exert control over the whole Black Sea.