KYIV (Reuters) - A U.S. plane carrying military equipment and munitions landed in Kyiv on Tuesday, the third shipment of a $200-million security package to shore up Ukraine as it braces for a possible Russian military offensive.

       "Our partners are increasing the amount of military assistance. And today we are meeting the third aircraft from the United States government as part of this assistance," Defence Minister Oleksii Reznikov told reporters before the plane landed.

       The United States has been Ukraine's most powerful backer in trying to avert a new attack by Russia, which has massed troops near Ukraine's border. Moscow denies planning an offensive.

       The United States has committed more than $650 million of security assistance to Ukraine in the past year and more than $2.7 billion in total since 2014, when Russia annexed Ukraine's Crimea peninsula.

       Ukraine has been a grateful recipient of the aid though it has criticised as "premature" a partial pullout of U.S. and other embassy staff.

       (Reporting by Natalia Zinets; writing by Matthias Williams, editing by Timothy Heritage)