KYIV, Dec 23 (Reuters) - Ukrainian forces have pulled out of the embattled eastern town of Siversk, Kyiv's military said on Tuesday, as Russian troops wage a battlefield offensive aimed at threatening key cities critical to Ukraine's defences in the east.The fall of Siversk comes with Ukraine under U.S. pressure to quickly negotiate peace in the nearly four-year-old war triggered by Russia's invasion, and puts Russian forces closer to the hub of Sloviansk, some 30 km (20 miles) to the west.The Reuters Daily Briefing newsletter provides all the news you need to start your day. Sign up here.Sloviansk is a...