KYIV, Jan 27 - Russia and NATO are continuing their military buildup around Ukraine, including more Russian fighter jets just across Ukraine's northern border in Belarus, CBS News reported.

Russia's ongoing military exercises with its Belarusian allies have fueled fears that Russia could launch an invasion into Ukraine within weeks.

The U.S. and its NATO allies presented President Vladimir Putin's government with a written response to his demands for "security guarantees" on Wednesday, hoping to keep the crisis on a diplomatic track.

On Thursday, Russia's foreign minister said Putin had personally reviewed the response hand-delivered by the U.S. ambassador in Moscow, CBS News reported. The reaction from Moscow indicated disappointment, but also a decision to keep talking, for now.

The biggest "guarantee" Putin's regime sought is a commitment that NATO will rule out admitting Ukraine or any of its neighbors as new members of the trans-Atlantic security alliance, while also pulling forces back from Russian borders. NATO has bolstered troop and weapons deployments in the region — as a direct response to Russia moving some 100,000 of its forces close to Ukraine's eastern frontier, and now to its north in Belarus.

The U.S. and NATO made it clear even before their formal response was delivered to Moscow that any ban on new NATO members in Eastern Europe was a "non-starter," but they indicated a willingness to discuss other issues, including military exercises, weapons deployments in the region and other "trust-building" actions that could be taken by both sides.

"There was no positive response to the main question," Russian Foreign Minister Sergey Lavrov said in a statement on Thursday, referring to Ukraine's bid for NATO membership, but "there is a response which gives hope for the start of a serious conversation on secondary questions."

But while diplomacy appeared set to continue, including bilateral talks expected in the coming days between Putin and France's leader, both sides in the standoff are keeping the pressure on.

On Thursday, Moscow accused the U.S. of planning to position more missiles in the region. A senior foreign ministry official warned that such steps risked sparking a "new missile crisis." His warning came weeks after Russia refused to rule out positioning military hardware in Venezuela or Cuba, which brought echoes of the 1962 Cuban missile crisis.

Six U.S. fighter jets arrived in Estonia, meanwhile, to take part in NATO military exercises. Slovakia's foreign minister said on Thursday that NATO was considering deploying additional troops to his country, too.

NATO military deployments like those — in what Russia considers its own backyard — are something the Biden administration has indicated a willingness to negotiate on.

"The placement of offensive missile systems in Ukraine, military exercises and maneuvers in Europe — all of these things would address, I think, mutual concerns, including concerns stated by Russia," Secretary of State Antony Blinken said on Wednesday as the U.S. response to Russia's demands was delivered. (cbs/ez)