US Air Force Gen Philip Breedlove told The Associated Press he wouldn't "write off involvement by any nation, to include the United States."
Foreign ministers of the 28-nation alliance have given Breedlove until Tuesday to propose steps to reassure NATO members nearest Russia that other alliance countries have their back.
"Essentially what we are looking at is a package of land, air and maritime measures that would build assurance for our easternmost allies," Breedlove told the AP. "I'm tasked to deliver this by next week. I fully intend to deliver it early."
In March, Russian troops took control of Ukraine's Crimean Peninsula, whose inhabitants then voted in a referendum to secede and join Russia. The US and other Western countries have accused Moscow of massing troops on Ukraine's border to maintain the pressure on the government in Kiev, and possibly for military use.
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Speaking at the end of a NATO conference in Paris, Breedlove told the AP the Russian armed presence near Ukraine's frontier continues unabated.
The Russians' assets include fixed and rotary wing aircraft, artillery, field hospitals, communications and jamming gear, he said.
Russian objectives remain unclear, the NATO commander said. The force could stand pat and intimidate Ukraine solely by its presence, drive south to create a land bridge with Crimea, push along the Black Sea coast to the Ukrainian port city of Odessa and the largely Russian Trans-Dniester enclave of Moldova, or invade areas of eastern Ukraine where ethnic Russians are also demanding unity with Russia, he said.