你所给的句子可以作以变形来理解——
If Russia should fail to make good on its Geneva commitments,America is promising further sanctions in the coming days .
原来给的整体句子实际就是【省略了If的虚拟条件句】;
is promising是【现在进行时,表示将来】.
另外,主句谓语动词 is promising 之所以这样用,是因为【承诺】这个动作在发此消息的阶段正在进行,即【目前或现阶段美国正在让人知晓其做出的承诺】.
祝你开心如意!