KYIV, Ukraine — Ukraine’s leaders are seeking to reassure the nation that a feared invasion from neighboring Russia is not imminent.

Those comments came even as they acknowledged the threat is real and prepared to accept a shipment of American military equipment Tuesday to shore up their defenses.

Russia has denied it is planning an assault, but it has massed an estimated 100,000 troops near Ukraine in recent weeks, leading the United States and its NATO allies to rush to prepare for a possible war.

Several rounds of high-stakes diplomacy last week failed to yield any breakthroughs, and this week tensions escalated further.

NATO said it was bolstering its deterrence in the Baltic Sea region, and the U.S. ordered 8,500 troops on higher alert.

Russian President Vladimir Putin has demanded that Ukraine be permanently barred from exercising its sovereign right to join the Western alliance and that other NATO actions, such as stationing troops in former Soviet bloc countries, be curtailed.

NATO has said the demands are unacceptable and that joining the alliance is a right of any country and does not threaten Russia.

Putin's critics argue that what he really fears is not NATO, but the emergence of a democratic, prospering Ukraine that could offer an alternative to Putin's increasingly autocratic rule, which Russians might find appealing.